CHEMACT is a computer code that uses the QRRK treatment of chemical activation reactions to estimate apparent rate constants for the various channels that can result in addition, recombination and insertion reactions. The working equations are developed, the necessary input file is described, and sources of required input data are discussed. Three applications of the code to representative examples of reactions relevant to combustion are presented. These include C2H5+ O2, H + C6H5Cl, and CH3+ OH. These reactions illustrate some of the expected behavior with different types of chemical activation and demonstrate the necessity of explicitly accounting for the effects of chemical activation in high temperature gas-phase reactions. Methods by which these results can be included in detailed kinetic mechanisms are discussed.