The results of a theoretical and experimental study of the effect of gas composition on the response of a thick‐source alpha gas densimeter are presented. A mathematical description of gauge response is developed and used to predict the response of a Po‐210 densimeter. Gauge response is measured for pure gases (He, CH4, N2, CO2, Ar) and gaseous mixtures having densities between 0.3 and 1.3 kg/m3. Qualitative predictions of the dependence of gauge response on gas composition, based on heuristic arguments, are borne out experimentally and theoretically. For a given density, response increases with increasing atomic number, and decreasing ratio of atomic number to atomic weight. The effect of gas composition on gauge response is more pronounced for lowZgases than for highZgases. The presence of hydrogen in a gas sample significantly decreases gauge response. Theoretical predictions show very good agreement with experimental measurements. The total uncertainty in gauge response is calculated for gases having atomic numbers nearZ=7. For a 2% uncertainty in average atomic number, the uncertainty in the density measurement is less than 2.5% at a density of 1 kg/m3.