Structures and isomerization reactions of the CH2CHCOOH isomers in S-0, T-1 and S-1 states have been studied with ab initio molecular orbital methods. The obtained results support a mechanistic model for the photochemical rearrangement of acrylic acid based on a non-radiative decay route. The CH2CHCOOH molecules are populated in the S-1 state by photo-excitation at 248 nm. Then the T-1 intermediate is formed via the S-1/T-1 intersystem crossing. A significant fraction of the CH2CHCOOH molecules in T-1 returns to the ground state by a decay through the T-1/S-0 crossing point. Vibrationally hot ground-state isomers are generated, which can undergo unimolecular decompositions.