The potential energy surfaces of the C-O cleavage, rotational isomerization, keto-enolic tautomerization, and dehydration reactions of acetylacetone in the lowest triplet and ground states have been determined using the complete active space self-consistent field and density functional theory methods. The main photochemical mechanism obtained indicates that the acetylacetone molecule in the S-2((1)pi pi*) state can relax to the T-1((3)pi pi*) state via the S-2-S-1 vibronic interaction and an S-1/T-1/T-2 intersection. The C-O fission pathway is the predominant dissociation process in the T-1((3)pi pi*) state. Rotational isomerization reactions proceed difficultly in the ground state but very easily in the T-1((3)pi pi*) state. Keto-enolic tautomerization takes place with little probability for acetylacetone in the gas phase.