Potential energy surfaces (PESs) of the maleic acid anion radical (MA(-center dot): cis isomer)/fumaric acid anion radical (FA(-center dot): trans isomer) system as a model system of their esters have been studied in detail using CASSCF method. The results suggest the following: The photoisomerization is initiated with the H-C-C-H dihedral angle distortion [hydrogen out of plain (HOOP) motion] on the D-1 PES. The C-C-C-C dihedral angle distortion occurs on the D-0 PES after the deactivation from D-1 to D-0. A large fraction of the net motion along the isomerization coordinate occurs on the D-0 PES. The D-0 state is responsible for the one-way nature of the photoisomerization.