The reaction of OH radical with cis- and trans-HONO has been investigated by ab initio molecular orbital and variational transition-state theory calculations. The overall mechanism has been elucidated and found to be quite complex. The bimolecular rate constants for the major reaction paths have been calculated and compared with existing, conflicting kinetic data. The total rate constant for the production of H2O and NO2 was concluded to have noticeable negative temperature dependence below 1000 K. The following two expressions are recommended for atmosphere and combustion modeling applications: k1 = 4.12 × 1012 (T/300) -0.8 cm3 mol-1 s-1, 200-500 K; k1 = 1.77 × 107 T1.5 exp [+1260/T] cm3 mol-1 s-1, 500-2000 K. The predicted rate constant for the low-temperature regime agrees quantitatively with the result of Burkholder et al. (Int. J. Chem. Kinet., 1992, 24, 711) measured at 298 > T > 373 K.