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<p>We present new models for the activity of iron (<span class="inline-formula"><i>γ</i>_Fe</span>) in solid face-centered cubic (fcc) and liquid FePt alloy at high temperature and pressure to facilitate their use as sliding buffer redox sensors under extreme conditions. Numerous experimental studies of <span class="inline-formula"><i>γ</i>_Fe</span> in FePt alloy at 100 kPa have produced a wide spread of values. By favoring high-temperature studies that are more likely to have produced equilibrium measurement and excluding experiments for compositions and temperatures that probably encountered ordered or unmixed low-temperature phases, we regress an asymmetric Margules activity–composition model with parameters <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>W</mi><mi mathvariant="normal">FePt</mi><mi mathvariant="normal">fcc</mi></msubsup><mo>=</mo><mo>-</mo><mn mathvariant="normal">121.5</mn><mo>±</mo><mn mathvariant="normal">2.1</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="102pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="f8f5a3ca8202d0ae7146ecdbddbb0df6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-789-2023-ie00001.svg" width="102pt" height="17pt" src="ejm-35-789-2023-ie00001.png"/></svg:svg></span></span> kJ mol<span class="inline-formula">⁻¹</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>W</mi><mi mathvariant="normal">PtFe</mi><mi mathvariant="normal">fcc</mi></msubsup><mo>=</mo><mo>-</mo><mn mathvariant="normal">93.3</mn><mo>±</mo><mn mathvariant="normal">4.3</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="96pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="7608fe4665a28f0f54fd6a73d3c441fe"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-789-2023-ie00002.svg" width="96pt" height="17pt" src="ejm-35-789-2023-ie00002.png"/></svg:svg></span></span> kJ mol<span class="inline-formula">⁻¹</span>. These values are close to the widely used model of Kessel et al. (2001), but for Pt-rich compositions they predict larger Fe activities and correspondingly more reduced oxygen fugacities. Activity–composition relations in liquid FePt are calibrated from direct measurements of activities and, most sensitively, from the trace of the Fe–Pt liquidus. Together, these yield asymmetric Margules parameters of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>W</mi><mi mathvariant="normal">FePt</mi><mi mathvariant="normal">liq</mi></msubsup><mo>=</mo><mo>-</mo><mn mathvariant="normal">124.5</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="76pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="1f613036c2091ea4f0a7b056ceff761a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-789-2023-ie00003.svg" width="76pt" height="18pt" src="ejm-35-789-2023-ie00003.png"/></svg:svg></span></span> kJ mol<span class="inline-formula">⁻¹</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>W</mi><mi mathvariant="normal">PtFe</mi><mi mathvariant="normal">liq</mi></msubsup><mo>=</mo><mo>-</mo><mn mathvariant="normal">94.0</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="a1e424de41f95836c15caba4c16401f1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-789-2023-ie00004.svg" width="70pt" height="18pt" src="ejm-35-789-2023-ie00004.png"/></svg:svg></span></span> kJ mol<span class="inline-formula">⁻¹</span>. The effects of pressure on both fcc and liquid FePt alloy are considered from excess-volume relations. Both solid and liquid alloy display significant positive excess volumes of mixing. Extraction of the excess volume of mixing for fcc FePt alloy requires filtering data for ordered low-temperature phases and corrections for the effects of magnetostriction on Fe-rich compositions which exhibit “Invar” behavior. Applied at high temperatures and pressures, both solid and liquid FePt alloys have strongly negative deviations from ideality at low pressure, which become closer to ideal at high pressure. These models provide a provisional basis for the calculation of <span class="inline-formula"><i>a</i>_Fe</span> in high-temperature, high-pressure experiments that, when combined with estimates of <span class="inline-formula"><i>a</i>_FeO</span>, allow characterization of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>f</mi><mrow class="chem"><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">2</mn></msub></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="15pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="3176f8ee00efdacde192a0f11f0b0d9a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-35-789-2023-ie00005.svg" width="15pt" height="14pt" src="ejm-35-789-2023-ie00005.png"/></svg:svg></span></span> under conditions relevant to magma oceans, core formation, and differentiation processes in the lower mantle of Earth or on other terrestrial planets. Improvements in these models require new constraints on the equation of state of FePt fcc alloy and documentation of the high-pressure melting relations in the system Fe–Pt.</p>