The mechanisms of photochemical rearrangement reactions are studied theoretically using a model system of bicyclic molecules that features a cyclopropane ring that is in proximity to a double bond moiety (such as CC or C O group), using the M06-L method and the 6-311G(d,p) basis set. The theoretical findings show that intersystem crossing is vital to successfully interpret the mechanisms. This theoretical research also shows that the photoisomerization mechanism should proceed as follows: Rea-S₀ → FC-T₁ → Min-T₁ → TS1-T₁ → T₁/S₀ → Int1-S₀ → TS2-S₀ → Int2-S₀ → TS3-S₀ → Pro-S₀. The theoretical evidence in this work confirms the experimental results for both [3.1.0] and [5.1.0] bicyclic molecules. The theoretical observations also predict that for both [2.1.0] and [4.1.0] systems, the quantum yields of the tautomeric keto forms are greater than those of the corresponding enol isomers.