Sensitivity and selectivity of polypyrrole (PPy) toward NH 3 , CO 2 , and CO have been studied at density functional theory (DFT). PPy oligomers are used both in the doped (PPy +) and neutral (PPy) form for their sensing abilities to realize the best state for gas sensing. DFT calculations are performed at the hybrid functional, B3LYP/6-31G(d), level of theory. Detection/interaction of CO is investigated from carbon [CO(1)] and oxygen termini of CO [CO(2)]. Interaction energies and charge transfer are simulated which reveal the sensing ability of PPy toward these gases. Furthermore, these results are supported by frontier molecular orbital energies and band gap calculations. PPy, in both the doped and neutral state, is more sensitive to NH 3 compared to CO 2 and CO. More interestingly, NH 3 causes doping of PPy and dedoping of PPy + , providing evidence that PPy/PPy + is an excellent sensor for NH 3 gas. UV−vis and UV− vis−near-IR spectra of nPy, nPy + , and nPy/nPy + −X complexes demonstrate strong interaction of PPy/PPy + with these atmospheric gases. The better response of PPy/PPy + toward NH 3 is also consistent with the experimental observations.