The kinetics and mechanisms of the HCO reactions with HONO and HNOH have been studied at the G2M level of theory based on the geometric parameters optimized at BH&HLYP/6-311G(d,p). The rate constants in the temperature range 200-3000 K at different pressures have been predicted by microcanonical RRKM and/or variational transition state theory calculations with Eckart tunneling corrections. For the HCO + HONO reaction, hydrogen abstraction from trans-HONO and cis-HONO by HCO produces H2CO + NO2, with the latter being dominant. Two other channels involving cis-HONO by the association/decomposition mechanism via the HC(O)N(O)OH intermediate, which could fragment to give H2O + CO + NO at high temperatures, were also found to be important. For the HCO + HNOH reaction, three reaction channels were identified: one association reaction giving a stable intermediate, HC(O)N(H)OH (LM2), and two hydrogen abstraction channels producing H2CO and H2NOH. The dominant products were predicted to be the formation of LM2 at low temperatures and H2NOH + CO at middle and high temperatures.