The reaction mechanism of HCHO with HO2radical has been studied at CBS-QB3 level of theory. Three direct hydrogen abstraction processes, one double hydrogen transfer mechanism, three cooperative hydrogen abstraction processes, and one additional channel have been identified for HCHO + HO2reaction. The calculated results indicate that the additional mechanism of HOCH2OO formation as well as the direct hydrogen abstraction process of HOC + H2O2formations is dominant. Other channels may be negligible due to the high barrier heights. Rate constants and branching ratios have been estimated by means of the conventional transition state theory with zero-curvature tunnelling over the temperature range of 275–1800 K. The calculation shows that the overall rate constant in the temperature of 275–1800 K is mainly dependent on the channel of HOCH2OO formation. The three-parameter expression for the total rate constant is fitted to becm3molecule−1s−1between 275 and 1800 K. [ABSTRACT FROM AUTHOR]