Involved in atmospheric, biological and industrial processes, ammonia must be monitored and controlled in order to limit health impact for human and livestock or defects in industrial productions. Polyaniline (PAni) based chemiresistive sensors are promising for ammonia detection at ppb level whereas no commercial sensors are able to detect this gas at low concentration. Yet, key parameters driving the performances of theses sensors are still not characterized. Current characterizations of PAni based gas sensors are primarily based on resistivity variation of the polymer composite as a function of pollutant concentration. This approach does not provide any information on the fundamental heterogenous interactions between the gas of interest and the polyaniline composite (PAni.C). Understanding these heterogenous interactions is essential for a better comprehension of material properties driving the performance and limitations of the sensors. To this end, a series of laboratory experiments were performed at room temperature in a Knudsen flow reactor, coupled with a modulated molecular beam quadrupole mass spectrometer for the real-time monitoring of the gas-phase. The objective of these experiments was twofold. The first fold is to provide a quantitative chemical characterization of PAni.C surface sites, applying the probe gas method. In this surface sites screening method, the solid is exposed to a series of gas molecules, to titrate the corresponding (acidic/basic/reductive) sites. Quantitative information is provided by determining the molecules of gas adsorbed per unit surface of the material, (Ns, molecules cm-2). Our results evidence that PAni.C surface is 100% composed by acidic sites. This result suggests that alkaline gas molecules, e.g. amines, could adsorb onto the surface and therefore act as interferent during ammonia detection. In a second fold, the adsorption of ammonia (NH3) and trimethylamine (TMA) on PAni.C was quantified through successive uptake experiments. In both cases, the uptake was found to follow, Langmuir type adsorption behaviour and results were fitted with the corresponding isotherm model, to define the partitioning coefficients (KLang cm3.molecule-1) and the maximum surface coverage (Nmax molecules.cm-2) of NH3 and TMA on PAni.C, respectively. From these data, Ns-NH3 and Ns-TMA, as a function of the concentration are determined for any gas concentration. Ns-NH3 value is higher than Ns- TMA at any concentration, suggesting different sensitivity to these gases. The uptake of NH3 and TMA was reversible, indicating that compounds are not chemisorbed on PAni.C. We propose that NH3/TMA are strongly bounded on PAni.C through hydrogen bonding, introducing a new approach in the formulation of gas sensors based on protonated polyaniline. Finally, Knudsen cell has appeared as a powerful tool to characterize fundamental heterogenous interaction taking place in chemiresistive gas sensors and identify potential interreferences.