AbstractTransport of CO2across polycarbonate films has been studied using a diffusion cell technique employing a radioactively labelled tracer (14CO2). Because the14CO2driving force could be established independently of the unlabelled CO2driving force, several classes of experiments not possible with conventional techniques were performed. These different classes of experiments showed measurably different time lags. Formally, these experiments all are limiting cases of the more general mixed‐gas permeation problem; however, simplifying assumptions in the dual sorption theory are possible because the tracer concentration approaches zero and because the two species in this special mixed‐gas problem exhibit the same dual sorption parameters. These simplifications allow derivation of analytical expressions for the time lag for both the unlabelled and labelled gas species. The experimental measurements are in good agreement with the dual sorption model formulated for the mixed‐gas tracer diffusion pr