In the present Letter, potential energy surfaces for the 1,5-H shift and alpha-bond cleavages in the S-0, T-1 and S-1 states have been investigated using the CASSCF, DFT and MP2 methods with the cc-pVDZ basis set, which provides some new insights into the mechanistic photochemistry of butanal. The 1,5-H shift proceeds mainly along the S, pathway, while the (alpha-C-C bond cleavage occurs as a result of intersystem crossing from S-1 to T-1. However, the alpha-C-H bond fission takes place with less probability. (C) 2002 Elsevier Science B.V. All rights reserved.