The reaction between HOCO and NO, and that between HOCO and O-2, have been examined using the quadratic configuration interaction method to locate and optimize the critical points on the potential energy surfaces. Analysis of the critical points provides new insight into new intermediates and pathways by which these reactions occur and help explain recent experimental results. In the HOCO+O-2 reaction, the symmetry-allowed products, CO2+HO2, can be obtained both via direct hydrogen abstraction by O-2 on the HCO radicals, as well as through an adduct, HOC(O)O-2, which can proceed to give the same products. The less-than-unity yield of CO2 observed in the experimental studies of the HOCO+NO reaction, as well as the lack of CO2 can be explained by the formation of a stable HOC(O)NO adduct. (C) 2004 American Institute of Physics.