The kinetics of the unimolecular decomposition of the n-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 (560-620 K) and bath gas density ((3-18) x 10(16) molecules cm(-3)) in two bath gases, He and N-2. The rate constants are in the falloff region under the conditions of the experiments. Structures, vibrational frequencies, and barriers for internal rotations of n-butyl and iso-butyl radicals and their decomposition transition states were obtained by ab initio calculations using UHF/6-31G* and MP2/6-31G* methods. The results of ab initio calculation, together with the reanalysis of earlier studies of the reverse reactions, were used to create transition-state models of the reactions of unimolecular decomposition of n-butyl (1) and iso-butyl (2) radicals. Falloff behavior of reaction 1 was reproduced using master equation modeling with the energy barrier height for decomposition obtained from optimization of the agreement between experimental and calculated rate constants. The values of [Delta E](all) = -28 cm(-1) (He) and -40 cm(-1) (N-2) for the average energy loss per collision were obtained using an exponential-down model. The resulting models of the reactions provide the high-pressure limit rate constants for the decomposition reactions (k(1)(infinity)(n-C4H9 --> C2H5 + C2H4) = 1.06 x 10(13) exp(-14005 K/T), k(2)(infinity)(iso-C4H9 --> CH3 + C3H6) = 2.14 x 10(12)T(0.65) exp(-15529 K/T) s(-1)) and the reverse reactions (k(-1)(infinity)(C2H5 + C2H4 --> n-C4H9) = 6.59 x 10(-21)T(2.44) exp(-2697 K/T), k(-2)(infinity)(CH3 + C3H6 --> iso-C4H9) = 1.66 x 10(-20)T(2.57) exp(-3879 K/T) cm(3) molecule(-1) s(-1)). Parametrization of the temperature and pressure dependence of the unimolecular rate constants for the temperature range 298-900 K and pressures 0.001-10 atm in He and N-2 is provided using the modified Lindemann-Hinshelwood expression.