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 H2O + 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 H2O + 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 + NO2. 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 x 10(-16)T(1.55) exp(-3328.5/T), k(OH) = 9.36 x 10(-14)T(0.86) exp(-2500.8/T), k(H2O) = 1.35 x 10(-17)T(1.89) exp(-1935.7/T), where the rate constant for H2O production represents the sum from both indirect displacement reactions.