The kinetics of the unimolecular decomposition of the sec-C4H9 radical has been studied experimentally in a heated tubular flow reactor coupled to a photoionization mass spectrometer. Rate constants for the decomposition were determined in time-resolved experiments as a function of temperature (598-680 K) and bath gas density ((3-18) x 10(16) molecules cm(-3)) in three bath gases : He, Ar, and Nz. The rate constants are in the falloff region under the conditions of the experiments. The results of earlier studies of the reverse reaction were reanalyzed and used to create a transition state model of the reaction. This transition state model was used to obtain values of the microcanonical rate constants, k(E). Falloff behavior was reproduced using master equation modeling with the energy barrier height for decomposition (necessary to calculate k(E)) obtained from optimization of the agreement between experimental and calculated rate constants. The resulting model of the reaction provides the high-pressure limit rate constants for the decomposition reaction (k(1)(infinity)(sec-C4H9 --> C3H6 + CH3) = 2.73 x 10(10)T(1.11) exp(- 15712 K/T) s(-1)) and the reverse reaction (k(-1)(infinity)(CH3 + C3H6 --> sec-C4H9)= 2.13 x 10(-19)T(2.28) exp(- 3319 K/T) cm(3) molecule(-1) s(-1)). Average values of [Delta E](down) = 363 (He), 447 (Ar), and 506 cm(-1) (N-2) for the average energy loss per deactivating collision were obtained using an exponential-down model. Parametrization of the temperature and pressure dependence of the unimolecular rate constant for the temperature range 298-1500 K and pressures 0.001-10 atm in He, Ar, and N-2 is provided using the modified Lindemann-Hinshelwood expression.