The potential energy surfaces of isomerization, dissociation, and elimination reactions for CH3CH2COCl in the So and S, states have been mapped with the different ab inito calculations. Mechanistic photodissociation of CH3CH2COCl at 266 nm has been characterized through the computed potential energy surfaces, the optimized surface crossing structure, intrinsic reaction coordinate, and ab initio molecular dynamics calculations. Photoexcitation at 266 nm leads to the CH3CH2COCl molecules in the S, state. From this state, the C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase. The barrier to the C-Cl bond cleavage on the S, surface is significantly increased by effects of the matrix and the internal conversion to the ground state prevails in the condensed phase. The HCl eliminations as a result of internal conversion to the ground T state become the dominant channel upon photodissociation of CH3CH2COCl in the argon matrix at 10 K.