The mechanism of the H + HONO reaction (for which no experimental data are available) has been elucidated by ab initio molecular orbital calculations using modified G2 and BAC-MP4 methods. These results indicate that the reaction occurs predominantly by two indirect metathetical processes. One produces OH + HNO and H 2 O + NO from the decomposition of vibrationally excited hydroxyl nitroxide, HN(O)OH, formed by H atom addition to the N atom of HONO. The other produces H 2 O + NO from the decomposition of vibrationally excited dihydroxylamino radical, N(OH) 2 , formed by H atom addition to the terminal O atom. These indirect displacement processes are much more efficient than the commonly assumed, direct H-abstraction reaction producing H 2 + NO 2 . A transition-state theory calculation for the direct abstraction reaction and RRKM calculations for the two indirect displacement processes give rise to the following rate constants, in units of cm 3 molecule -1 s -1 for the 300-3500 K temperature range under atmospheric conditions: k H2 = 3.33 × 10 -16 T 1.55 exp(-3328.5/T), k OH = 9.36 ×10 -14 T 0.86 exp(-2500.8/T), K H2O = 1-35 × 10 -17 T 1.89 exp-(-1935.7/T), where the rate constant for H 2 O production represents the sum from both indirect displacement reactions.