The cis-trans isomerization of azo chromophores covalently bound to semicrystalline polymer films was investigated. Various azo chromophores were introduced onto the functionalized semicrystalline polymer films by a chemical transformation reaction and their isomerization behavior was investigated by UV-visible spectroscopy and a contact-angle goniometer. The thermal Z --> E isomerization rate of the azo moiety covalently bound to low-density polyethylene (LDPE) and polypropylene (PP) films was determined. The rate of thermal isomerization was also compared between amorphous and semicrystalline polymer films bearing azo chromophores. The effect of the intramolecular steric factor and the nature of chromophores was studied on the thermal isomerization rate of the polymer films.