Quantum chemical Gaussian-3 (G3), and complete basis set (CBS) composite energy methods are applied to investigate tert-butyl radical decomposition reaction kinetics and energetics. The available experimental thermodynamic and kinetic data are employed to assess the accuracy of these calculations. The CBS method proves to have good agreement with the experimental data, indicating it is a good method for studying other hydrocarbon cracking reactions involving large species. A kinetic model of the reaction with pressure and temperature effects is also proposed. For P <= P-0, k [s(-1)] = (3.93 X 10(12)) X P-0.35 X e((-17878.5/T)); for P > P-0, k [s(-1)] = (2. 0 X 10(13)) X e((-18096.0/T)), where P is in units of kPa, T in units of Kelvin, and P-0 = (1.04 X 10(2)) X e((-621.43/T)). The advantage of this simple kinetic model is that it can be easily applied to different reaction conditions without performing additional costly calculations. (c) 2006 American Institute of Chemical Engineers.