We report the first measurements of the effect of ammonia gas on the temperature dependence of the conductivity and thermoelectric power of polypyrrole films. Our data are for samples of very different conductivities, extending down to a temperature of 200 K for low-conductivity polypyrrole gas sensors, and down to 4.2 K for more highly-conducting PPy(PF6) samples. We demonstrate that (except for the most metallic case) our polypyrrole samples show greater sensitivity to ammonia as the temperature is lowered (i.e. the fractional reduction in conductivity is greater at lower temperatures). Remanent decreases in conductivity are present after the removal of ammonia for higher pressure exposures, and remanent increases in the metal-like thermoelectric power for the PPy(PF6) for samples grown at higher temperatures. Our results indicate that the mechanism of this conductivity decrease in PPy(PF6) is that ammonia causes a reduction in the size of metallic regions as disordered barrier regions are thickened. (c) 2006 Wiley Periodicals, Inc.