Solubility and diffusivity measurements of a probe gas (CO2), which has an inherently low solubility in the polymer, have been used to characterize residual porosity in polymeric latex films. Sorption isotherms resembling those of a glassy polymer were obtained, even though the glass transition temperature of the polymer was 1°C, about 30°C below the experimental temperature. Solvent cast films of the same polymer exhibited much lower solubilities, and followed the expected Henry's law behavior. CO2 solubility and diffusivity were found to decrease with aging time for the latex films, but did not quite reach the values of the solvent cast films, even after 75 days at room temperature. The sorption data could be described by the dual-mode sorption model, which is commonly employed in the analysis of glassy polymer sorption isotherms. Estimates of the amount of porosity were made from the sorption data, and values ranging from 0.6–0.03% were obtained for latex films aged from 62 h to 75 days, respectively. Our results suggest that permeability differences noted by others for latex and solvent cast films of the same polymer are due to the substantial solubility differences for low-solubility penetrants in these two types of films.