The use of polyaniline and especially sulfonated polyaniline (SPAN) is explored for sensing carbon dioxide (CO2) at room temperature. Frequency-dependent AC measurements were carried out to detect changes in impedance of the polymer, drop casted on interdigitated electrodes, when exposed to CO2 gas. Aqueous pH buffers were first used to study pH-dependent conductivity to identify a good candidate polymer for CO2 sensing. Emeraldine-base polyaniline exhibits only a significant conductivity change below pH 4 whereas SPAN shows also a conductivity change between pH 4 and 7. This makes SPAN more suitable for CO2 sensing than emeraldine-base polyaniline. The impedance of SPAN films is found to decrease when exposed to CO2 concentrations above 20,000 ppm. Blending SPAN with polyvinyl alcohol further increases the sensitivity for CO2.