CH3COCH3 photodissociation was investigated using the CASSCF energy gradient techniques. After the acetone molecules are populated in the S-1 state by photoexcitation at 193 nm, the intersystem crossing to the T-1 surface is the most probable pathway for CH3COCH3 (S-1) deactivation. Relaxing to the T-1 state, CH3COCH3 (T-1) first dissociates into H3CO ((2)A') and CH3 ((2)A(2)") products, and then the CH3CO ((2)A') formed further easily dissociates into CH3 and CO. This stepwise mechanism is consistent with numerous experiments.