Acrylic acid and HCl-doped polyanilines were synthesized by chemical oxidative polymerization. The synthesized materials were used as sensors for ammonia. Comparison of the responses of the two polymers reveal that the acrylic acid-doped polymer exhibits higher sensitivity and reversibility. Further, the resistance is observed to decrease on exposing the acrylic acid-doped polyaniline to saturated ammonia vapors. A reversed trend is observed in the case of HCl-doped polyaniline. The results are explained in terms of the differences in the chemical interactions of the two polymers with respect to ammonia vapors. The proposed mechanism is further supported by the X-ray diffraction and FTIR analysis. The X-ray diffractogram of acrylic acid-doped polymer shows an enhancement in the crystallinity on exposure to ammonia vapors, while the HCl-doped polymer exhibits a loss in crystallinity. The FTIR spectra shows a higher doping level in acrylic acid doped polymer as observed from the intense peak of the dopant ion at 1158 cm(-1), which is seen to be shifted to a lower wavenumber i.e. similar to 1128 cm(-1) on exposing the polymer to ammonia vapors. On the other hand, in HCl-doped polyaniline, the peak of the dopant ion similar to 1120 cm(-1) is initially less intense, which is further suppressed on exposure to ammonia. Conductivity measurements show a large vapor-induced increase in conductivity, in the case of ammonia-exposed acrylic acid-doped polyaniline, which results in the formation of a more crystalline-conducting phase. Exactly the opposite results were obtained in the case of HCl-doped polyaniline exposed to ammonia.