The unimolecular reactions of the 1-butoxy radical, a prototype of atmospherically important alkoxy radicals, are studied. The structures of the isomers of the C4H9O radical and the transition structures of the isomerization and main decomposition reactions of 1-butoxy are determined by ab initio quantum chemical methods. The energy-dependent rate coefficients of the unimolecular reactions involved were obtained in RRKM calculations. The threshold energy for the fastest channel, the 1,5 intramolecular H-atom transfer is 8.12, 9.23, 9.16, and 9.51 kcal mol(-1) according to the MP-SAC2, BAC-MP4, DFT, and an additively corrected MP4 method, respectively. For the beta-C-C dissociation reaction we found a barrier of 16.03, 13.34, and 12.39 kcal mol(-1) at the BAC-MP4, DFT, and additively corrected MP4 level. The barrier height for the 1,4 isomerization is close to that of the C-C bond rupture. The fastest reaction of the radical is isomerization involving a six-member transition structure, converting the alkoxy radical into an OH-substituted alkyl radical. The decomposition reaction can compete with the 1,5 isomerization step at high excitation or at combustion temperatures.