The initiation process of the thermal isomerization reaction from methyl 4-(dimethylamino)benzenesulfonate (MDBS) to N,N,N-trimethylbenzenaminium-4-sulfonate (TBS) in the crystalline state has been studied by theoretical calculations : Structural optimization of possible ionic intermediates and the transition-state structure has been carried out by ab initio MO calculations. The comparison of the obtained transition-state structure with the molecular arrangement in the MDBS crystal indicates that cooperative rotation of molecules around their molecular long axes coupled with structural deformation of the methoxy group in an MDBS molecule is necessary for the topochemical initiation reaction for the thermal isomerization in the crystal.