The mechanism of the radical-radical reaction C2H3 + CH3 (1) was studied by quantum chemical methods. The pathways of reaction channels observed in previous experimental studies, as well as those of other potential channels, were investigated. The results of the quantum chemical study and of the earlier experimental work were used to create a model of the chemically activated route (C2H3 + CH3 reversible arrow C3H6 --> H + C3H5) of reaction 1. In this model, energy- and angular momentum-dependent rate constants are calculated using the RRKM method in combination with the microcanonical variational selection of the transition states. Pressure effects are described by solution of the master equation. Temperature and pressure dependences of the rate constants and product yields were investigated. The model was used to predict the rate constants and branching fractions of reaction I at temperatures and pressures outside the experimental ranges. The same model was used to analyze kinetics of two other reactions which occur on the same potential energy surface: the thermal decomposition of propene (2) and the reaction of H atom with allyl radical, H + C3H5 reversible arrow C3H6 + C2H3 + CH3 (3). The results demonstrate the increasing importance of the CH3 + C2H3 channels in both reactions 2 and 3 at high temperatures (above similar to1500 K).