A variety of 4,4'-substituted azobenzenes has been synthesised and the kinetics of the thermal cis-trans-relaxation of the substances was studied in detail in solution and embedded in a poly(methyl methacrylate) (PMMA) matrix by UV-Vis spectroscopy. Considerable differences in relaxation times were found for the various azobenzenes which interestingly could not be fully explained by simply comparing the substituents with regard to their electron-donating or -withdrawing nature. Any substituents, especially very polar ones, increase the thermal cis-trans-reaction rate. Some of the chromophores were covalently attached to a PMMA-copolymer and we found that this significantly slowed down the isomerisation kinetics compared to the embedding of low molar mass azobenzenes in a polymer matrix. But our study showed also that, even at room temperature, the thermal cis-trans-relaxation of 4,4'-substituted azobenzene chromophores can never be fully suppressed, but only slowed down. (c) 2012 Elsevier Ltd. All rights reserved.