Ab initio generalized Born (GB) method including solvent effects based on the GB formula was applied to two push-pull diazenes, methylcyanodiazene (MCD) and dimethylaminonitrodiazene (DAND), and the solvent effect on the cis-trans isomerization reaction was examined. In both MCD and DAND, differential solvation was recognized clearly; the transition state (TS) of the rotation mechanism was stabilized most by solvent. However, in the case of MCD, the TS for the inversion of the pull group was expected to be lower in energy than the rotation TS in aqueous solution. In order to analyze the solvent effect on the electronic structure and energy of the cis and trans forms and of TS, the solvation energy was divided into the polarization energy of the solute molecule and the solvation energy of the polarized solute molecule. The magnitude of these energy components was in the order, rotation TS > inversion TS > cis similar to trans. The group charge population of the donor and acceptor groups was used to account the charge separation at each TS, which was strongly related to the stabilization by solvent. (C) 2001 Elsevier Science B.V. All rights reserved.