The mechanisms of the photochemical isomerization reactions were investigated theoretically by using a model system of 1,2-dihydro-1,2-azaborine with the CAS(6,6)/6-311G(d,p) and MP2-CAS-(6,6)/6-311++G(3df,3pd)//CAS(6,6)/6-311G(d,p) methods. Three reaction pathways, which lead to three kinds of photoisomers, have been examined. The structures of the conical intersections, which play a decisive role in such photorearrangements, were obtained. The thermal (or dark) reactions of the reactant species have also been examined by using the same level of theory to assist in providing a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1,2-dihydro-1,2-azaborine, which leads to the Dewar 1,2-dihydro-1,2-azaborine photoproduct, is as follows: reactant→Franck–Condon region→conical intersection→photoproduct. The results obtained allow a number of predictions to be made.