The kinetics and mechanism of the H + HNO3 reaction have been elucidated with ab initio molecular orbital and statistical theory calculations. Our room temperature reaction rate results accord well with available experimental data. The reaction is dominated by an indirect metathetical process taking place via vibrationally excited dihydroxyl nitroxide, ON(OH)2, producing OH + cis-HONO. The excited ON(OH)2 also undergoes molecular elimination, yielding H2O + NO2 as a minor competing reaction. The direct H abstraction reaction forming H2 + NO3 was found to be the least important one. At atmospheric pressure, we recommend the following expressions for the three rate constants, in units of cm3/molecule s, from the 300-3000 K temperature range for H + HNO3 collision yielding the products H2 + NO3 by direct mechanism ka = (9.24 × 10-16)T1.53e-8253/T based on CTST calculations, OH + cis-HONO by indirect mechanism kb = (6.35 × 10-19)T2.30e-3511/T, and H2O + NO2 by indirect mechanism kc = (1.01 × 10-22)T3.29e-3163/T, the latter two are based on Arrhenius fits to the solution of the master equation which includes RRKM microscopic rate constants and tunneling corrections.