The electronic mechanism of the photoisomerization of the dicyanoacetylene complex of platinum Pt(PH3)(2)(C4N2) to the acetylide complex Pt(PH3)(2)(CN)(C=CCN) has been investigated theoretically. The geometries of the ground and excited states are optimized by the Hartree-Fock (HF) and single excitation configuration interaction (SE-CI) methods, respectively. In the thermal process, the decomposition reaction of Pt(PH3)(2)(C4N2) into Pt(PH3)(2) and C4N2 occurs preferentially but the association leads to the acetylene complex rather than to the acetylide complex. The reactant, transition state, and product all have planar structures. On the other hand, in the photochemical process, the dicyanoacetylene complex is isomerized smoothly into the acetylide complex. Neither the decomposition of Pt(PH3)(2)(C4N2) into the neutral separated system Pt(PH3)(2) + C4N2 nor that into the ionic separated systems Pt(PH3)(2)(+) + C4N2- and Pt(PH3)(2)(C4N2)(+) + CN- occurs. The intramolecular photoisomerization occurs through a bisect complex in the singlet excited state and a three-coordination Pt complex in the triplet state.