AbstractMeasurements of the effective self‐diffusion coefficients for carbon dioxide and methane were performed in the transition pressure range. The experiments utilized carbon‐14 tagged gases and semiconductor radiation detectors in a transient type of experiment. A detailed analysis of the errors associated with the experiments indicated that the accuracy of the measurements was near 5%. The results verify that the additive resistance law is valid for the representation in the transition range. The unknown effects of surface diffusion make the values to be employed in this law subject to review. As an example one surface diffusion model is conside