This contribution presents results of kinetic studies of the isomerization of photochromic 4-fluoro-4'-methoxyazobenzene dissolved in the liquid-crystalline pentylcyanobiphenyl (5CB). Deviations from the expected Arrhenius behavior were observed for the thermal cis-trans reaction near the phase transition temperature of 5CB. The activation energy of the thermal cis-trans isomerization (92 kJ/mol) and the preexponential factor (1.5 x 10(11) s(-1)) are lower than the respective values determined for the same dye dissolved in common solvents, apparently scaling with the polarity of the solvent. It was also found that the trans-cis isomerization results in the appearance of the isotropic phase at temperatures substantially lower than the thermodynamic nematic-isotropic phase transition temperature. A model, relating the kinetics of the isomerization to characteristic times of the phase transition has been put forward. The model takes into account differences in the solubility of both forms in the liquid-crystalline matrix, resulting from different compatibilities of their molecular shapes with that of 5CB.