The reduction of HONO by HNO and NH3 has been investigated by means of ab initio molecular orbital and transition-state theory (TST) calculations. The main reaction channels for the HNO + trans-HONO (cisHONO) reactions are those proceeding via five-member ring transition states, leading to the production of NO and H2O. In the temperature range 300-1000 K, TST calculations predict an A factor of 2.25 × 1010 cm3 mol-1 s-1 (or 3.63 × 1010 cm3 mol-1 s-1) and an apparent activation energy of 20.9 kcal/mol (or 21.9 kcal/mol) for the HNO + trans-HONO (or m-HONO) reaction. In the NH3 + HONO system, the reaction NH3 + cis/trans-HONO → H2NNO + H2O with barrier heights centering around 34 kcal/mol can occur at high temperatures. The reversible H-atom exchange reaction NH3 + cis-HONO ⇔ H2NH-O(H)NO ⇔ NH2H + cis-HONO occurs readily. The calculated rate constant for the reaction at 300 K is 1.06 × 106 cm3 mol-1 s-1, in reasonable agreement with the experimental value of 2.2 × 106 cm3 mol-1 s-1.