Quantification of the rate of gas exchange across the air-water interface is essential in understanding the biogeochemical cycling of carbon in mountain streams. However, estimating the gas transfer velocity (k) is not trivial, due to high turbulence and subsequent bubble-mediated gas transfer. Schmidt scaling is often used to estimate gas transfer velocities of climate relevant gases (e.g. CO2) from tracer gases (e.g. argon (Ar)), but this method has high uncertainty when scaling between gases of different solubilities in streams with bubble-mediated gas transfer. Here we explore a method for the estimation of gas exchange of CO2 from Ar by performing dual tracer gas additions in mountain streams. Ar and CO2 gas were simultaneously and continuously injected into streams and gas exchange rates were estimated using an exponential decline model. The mean ratio of gas exchange of Ar to CO2 (a) was 1.7 (95% credible interval of 1.3 to 2.3), approximately equal to the theoretical value of 1.7 (based both on Schmidt scaling and solubility). This result indicates that Ar can be used to estimate gas transfer of CO2 with scaling but with some uncertainty. Finally, modeled results suggest that the use CO2 as a tracer gas to measure gas exchange in streams with environmental conditions favoring interconversion to bicarbonate (i.e, high pH and alkalinity), can result in an overestimation of the gas transfer velocity k.