The mechanisms of photochemical isomerization reactions were investigated theoretically using the following model systems: spiro[2,4]hept-1-ene, 1-vinylcyclopentene, and vinylidenecyclobutane with the CASSCF/6-311G(d) (six-electron/six-orbital active space) and the MP2-CAS/6-311G(d,p)/CASSCF/6-311G(d) methods. The structures of the conical intersections, which play a central role in such photorearrangements, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations strongly indicate that a vinylcarbere-like intermediate can serve as a basis for discussion and rationalization of the results of the above photoisomerizations. Besides this, our theoretical findings suggest that the conical intersection mechanism found in this work gives a good explanation and supports the experimental observations. Also, all the relative final product yields predicted in the present work are in good agreement with the available experimental findings.