The potential energy surfaces of dissociation and elimination reactions for CH3COCl in the ground (S-0) and first excited singlet (S-1) states have been mapped with the different ab inito calculations. Mechanistic photodissociation Of CH3COCl has been characterized through the intrinsic reaction coordinate and ab initio molecular dynamics calculations. The (alpha-C-C bond cleavage along the S, pathway leads to the fragments of COCl((2)A") and CH3 ((2)A') in an excited electronic state and a high barrier exists on the pathway. This channel is inaccessible in energy upon photoexcitation of the CH3COCl molecules at 236 nm. The S-1 alpha-C-Cl bond cleavage yields the Cl(P-2) and CH3CO((X) over tilde (2)A ') fragments in the ground state and there is very small or no barrier on the pathway. The S-1 alpha-C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase, followed by CH3CO decomposition to CH3 and CO. The barrier to the C-Cl bond cleavage on the S-1 surface is significantly increased by effects of the argon matrix. The S-1 alpha-C-Cl bond cleavage in the argon matrix becomes inaccessible in energy upon photoexcitation of CH3COCl at 266 nm. In this case, the excited CH3COCl(S-1) molecules cannot undergo the C-Cl bond cleavage in a short period. The internal conversion from S-1 to S-0 becomes the dominant process for the CH3COCl(S-1) molecules in the condensed phase. As a result, the direct HCl elimination in the ground state becomes the exclusive channel upon 266 nm photodissociation of CH3COCl in the argon matrix at 11 K.