Two polycaprolactam films were prepared characterized by different crystallinity degrees, the free volume model being prevalent in the former and the pore model in the latter. They were colored with azo dyes XC6H4N[DOUBLE BOND]NC6H4N(C2H5)2 (where X is H and COOH), the latter of which gives specific interactions with the polymer. The kinetics of the thermal cis–trans isomerization of these dyes has been studied below and above the glass transition temperature of the polymer (Tg) in the range 20–80°C. In every case, the isomerization process was found to be the result of a multiplicity of first-order reactions and the dependence on temperature exhibited a break at the Tg value. Lightfastness of the dyes in the polymer matrices has been also investigated. The isomerization and fading kinetics are discussed in terms of polymer structural features as well as of the nature of the dye.