Gas-phase reactions of HONO with species of atmospheric interest, NO2, O-3, and HCl, have been investigated by means of ab initio molecular-orbital and transition-state theory calculations. For the HONO + NO2 reaction, the most favorable pathway is found to be the abstraction of OH in HONO by NO2, leading to the formation of HNO3 and NO products. The activation energies computed at the G2M (RCC,MP2) level of theory for the abstraction from trans- and cis-HONO are predicted to be 31.6 and 33.4 kcal/mol, respectively. At 300 K, the calculated rate constant is smaller than 1.2 x 10(-14) cm(3) mol(-1) s(-1). For the HONO + O-3 reaction, the barrier height for the production of HNO3 + O-2 is predicted to be 15.0 kcal/mol with the rate constant at 300 K, 5.7 x 10(-2) cm(3) mol(-1) s(-1). For the trans- and cis-HONO + HCl --> H2O + ClNO reactions, our G2M calculations predict the corresponding barrier heights of 12.9 and 14.4 kcal/mol with predicted rate constants, 1.3 x 10(2) and 0.13 x 10(2) cm(3) mol(-1) s(-1) at 300 K. The results of our theoretical modeling indicate that the three gas-phase reactions are too slow to be significant under the stratospheric and tropospheric conditions.