AbstractElementary bimolecular processes that involve formation of a chemically activated intermediate species are common. We address the general problem of modeling these processes and describe the necessary and sufficient information that must be specified to assure that a kinetics model will extrapolate the rate constants for those reactions over wide ranges of temperature and pressure. The approach is illustrated for the system centered around the HOCO intermediate. Here, specification of the temperature and pressure dependence of three rate constants,k(T,P) and the temperature dependence of two equilibrium constants,K(T), is necessary and sufficient, viz:Rate constants are cast in the form of an analytical expression, suggested by Troe, and appropriate parameters are tabulated.