Interest for air quality grows and sensors emerge as a promising technology to empower citizen and address air quality. Ammonia (NH3) is a gas of special concern for air quality as it is involved in several processes: atmospheric, industrial, and biological. Conductive polymer (CP) gas sensors are appealing due to affordability, tunability, sensitivity and low power-consumption. However, these chemiresistive sensors are limited by their stability, selectivity and dependence on relative humidity (RH). Indeed, interactions between water molecules and CP show multiple impacts on sensor response through: (i) CP doping, (ii) charge mobility enhancement, (iii) modification of gas uptake, etc. This work aims to provide comprehensive understanding of the role of H2O on the uptake of NH3 on a specific CP gas sensor: doped polyaniline (dPAni). First, the amount of H2O in the material structure is quantified for a wide range of atmospheric relevant RH conditions (20 % to 70 %) at 25°C. Second, the number of NH3 molecules taken up by dPAni is quantified as a function of RH (1 % to 50 %). In the meantime, the electrical response of dPAni sensor to NH3 is recorded. Results reveal that H2O content in dPAni increases linearly from 1 to 10 wt% with increasing RH. Moreover, uptake experiments evidence that the number of NH3 molecules taken up on dPAni increases by a factor of 100 from 1 to 50 % RH conditions. These results allow correlating the uptake of NH3 on dPAni sensor with corresponding electrical response. Finally, based on quantitative NH3 uptake data and specific surface area of dPAni, the contribution of water molecule to NH3 diffusion in the CP is identified. This result unravels the role of water in the interaction mechanism underlying the sensing of NH3 by dPAni.