Isomerization of azobenzene has been studied for many years, but some aspects are still unclear or controversial. This work provides further insight and clarifies the role that environmental parameters play in determining the reaction kinetics of azobenzene derivatives in solution. Detailed spectroscopic measurements of the photoisomerization and thermal isomerization kinetics in mixtures of various solvents and polystyrene have been carried out. The results indicate a strong dependence of both photoisomerization and thermal isomerization rates on the polarity, but not on the viscosity of the solvent.