The mechanistic aspects of the photo induced reactions of gaseous amyl nitrite have been investigated using ab initio calculations. We show that the C5H11O-NO bond dissociation mechanism is different on the S-1 and S-2 surfaces, respectively. The subsequent conformational changes in the ground state 1-pentoxy radical involves several barriers. Interestingly, we show that the intramolecular 1,5-H transfer in the 1-pentoxy radical proceeds in the opposite direction on the S-1 state compared to the S-0 reaction. Moreover, the results show that the excited state reaction of the alkoxy radical may be a proton transfer that proceeds on a repulsive surface. (C) 2009 Elsevier B.V. All rights reserved.