A mechanistic study of the trans reversible arrow cis isomerization of the azobenzene moiety in a side-chain liquid-crystal polymer system was carried out with six liquid-crystalline polymethacrylates in which different electron-withdrawing substituents were attached to the para-positions of the azobenzene chromophores. Compared to the non-nitro-substituted azo polymers, the nitro-substituted azo polymers exhibited two quite different behaviors: an extraordinarily high reaction rate of the thermal cis-trans isomerization and an unexpected composition of cis-trans isomers obtained from the photochemical trans-cis isomerization process. A potential energy profile for the isomerization process was established on basis of the structures of the proposed transition states and was employed to elucidate the reaction mechanism. The results confirmed that the nitro-substituted azo polymer system proceeded via a rotation mechanism in either direction of the trans reversible arrow cis isomerization reaction, whereas the non-nitro-substituted species were more likely to follow an inversion mechanism.