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Gas exchange is a parameter needed in stream metabolism and trace gas emissions models. One way to estimate gas exchange is via measuring the decline of added tracer gases such as sulfur hexafluoride (SF₆). Estimates of oxygen (O₂) gas exchange derived from SF₆ additions require scaling via Schmidt number (<i>Sc</i>) ratio, but this scaling is uncertain under conditions of high gas exchange via bubbles because scaling depends on gas solubility as well as <i>Sc</i>. Because argon (Ar) and O₂ have nearly identical Schmidt numbers and solubility, Ar may be a useful tracer gas for estimating stream O₂ exchange. Here we compared rates of gas exchange measured via Ar and SF₆ for turbulent mountain streams in Wyoming, USA. We measured Ar as the ratio of Ar  :  N₂ using a membrane inlet mass spectrometer (MIMS). Normalizing to N₂ confers higher precision than simply measuring [Ar] alone. We consistently enriched streams with Ar from 1 to 18 % of ambient Ar concentration and could estimate gas exchange rate using an exponential decline model. The mean ratio of gas exchange of Ar relative to SF₆ was 1.8 (credible interval 1.1 to 2.5) compared to the theoretical estimate 1.35, showing that using SF₆ would have underestimated exchange of Ar. Steep streams (slopes 11–12 %) had high rates of gas exchange velocity normalized to Sc = 600 (<i>k</i>600, 57–210 m d⁻¹), and slope strongly predicted variation in <i>k</i>600 among all streams. We suggest that Ar is a useful tracer because it is easily measured, requires no scaling assumptions to estimate rates of O₂ exchange, and is not an intense greenhouse gas as is SF₆. We caution that scaling from rates of either Ar or SF₆ gas exchange to CO₂ is uncertain due to solubility effects in conditions of bubble-mediated gas transfer.