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The boron isotope composition (<i>δ</i>¹¹B) of marine biogenic carbonates has been predominantly studied as a proxy for monitoring past changes in seawater pH and carbonate chemistry. However, a number of assumptions regarding chemical kinetics and thermodynamic isotope exchange reactions are required to derive seawater pH from <i>δ</i>¹¹B biogenic carbonates. It is also probable that <i>δ</i>¹¹B of biogenic carbonate reflects seawater pH at the organism's site of calcification, which may or may not reflect seawater pH. Here, we report the development of methodology for measuring the <i>δ</i>¹¹B of biogenic carbonate samples at the multi-collector inductively coupled mass spectrometry facility at Ifremer (Plouzané, France) and the evaluation of <i>δ</i>¹¹B_CaCO₃ in a diverse range of marine calcifying organisms reared for 60 days in isothermal seawater (25 °C) equilibrated with an atmospheric <i>p</i>CO₂ of ca. 409 µatm. Average <i>δ</i>¹¹B_CaCO₃ composition for all species evaluated in this study range from 16.27 to 35.09 ‰, including, in decreasing order, coralline red alga <i>Neogoniolithion</i> sp. (35.89 ± 3.71 ‰), temperate coral <i>Oculina arbuscula</i> (24.12 ± 0.19 ‰), serpulid worm <i>Hydroides crucigera</i> (19.26 ± 0.16 ‰), tropical urchin <i>Eucidaris tribuloides</i> (18.71 ± 0.26 ‰), temperate urchin <i>Arbacia punctulata</i> (16.28 ± 0.86 ‰), and temperate oyster <i>Crassostrea virginica</i> (16.03 ‰). These results are discussed in the context of each species' proposed mechanism of biocalcification and other factors that could influence skeletal and shell <i>δ</i>¹¹B, including calcifying site pH, the proposed direct incorporation of isotopically enriched boric acid (instead of borate) into biogenic calcium carbonate, and differences in shell/skeleton polymorph mineralogy. We conclude that the large inter-species variability in <i>δ</i>¹¹B_CaCO₃ (ca. 20 ‰) and significant discrepancies between measured <i>δ</i>¹¹B_CaCO₃ and <i>δ</i>¹¹B_CaCO₃ expected from established relationships between abiogenic <i>δ</i>¹¹B_CaCO₃ and seawater pH arise primarily from fundamental differences in calcifying site pH amongst the different species. These results highlight the potential utility of <i>δ</i>¹¹B as a proxy of calcifying site pH for a wide range of calcifying taxa and underscore the importance of using species-specific seawater-pH–<i>δ</i>¹¹B_CaCO₃ calibrations when reconstructing seawater pH from <i>δ</i>¹¹B of biogenic carbonates.