Experimental literature data on the chemically and thermally activated decomposition of sec-C4H9 radical were analyzed by weak collision master equation modeling. A reaction model with very little flexibility in its properties was created on the basis of ab initio calculations and experimental kinetic and thermochemical data. Rate constants and branching fractions for the:chemically activated reaction were calculated using the virtual component formalism. The resultant model quantitatively describes (1) data on the stabilization-to-decomposition ratios as functions of temperature and pressure obtained in experiments using H + butene reactions as the sources of chemical activation and (2) experimental data on the thermal decomposition bf sec-C4H9 radicals. Values of (DeltaE)(down), the average energy transferred per deactivating collision, derived from modeling of experimental data demonstrate strong positive temperature dependencies for a variety of bath gases. Qualitative shapes of falloff curves for the chemically activated reaction were analyzed. Comparison of the weak collision model with the results of the modified strong collision treatment demonstrates the inadequacy of the latter.