Bimolecular QRRK (Quantum Rice-Ramsperger-Kassel) analysis is a simple method for calculating rate constants of addition and recombination reactions, based on unimolecular quantum-RRK theory. Input parameters are readily derived, and rate constants and reaction branching can be predicted with remarkable accuracy. Such predictive power makes the method especially useful in developing mechanisms of elementary reactions. Furthermore, from the bimolecular QRRK equations, limiting forms of the rate constants in the limits of low and high pressure are developed. Addition/stabilization is pressure-dependent at low pressure but pressure-independent at high pressure, as is conventionally understood for simple decomposition, its reverse. In distinct contrast, addition with chemically activated decomposition has the opposite behavior: pressure independence at low pressure and pressure dependence [as (pressure)−1] at high pressure. The method is tested against data and illustrated by calculations for O + CO → CO2; for H + O2 → HO2 or O + OH; for H + C2H4 → C2H5 or C2H3 + H2; and for H + C2H3 → C2H4 or H2 + C2H2.