AbstractRandom copolymers ofN‐vinylcarbazole (VC) and alkylstyrenes with variable molar content of VC (first series) or variable size of alkyl substituent (second series) were characterized by the heat capacity measured at normal pressure in the temperature interval 203–523 K, as well as by measurements of specific volume and thermal diffusivity in the temperature interval 303–523 K and in the pressure range 30–100 MPa. It was established that the increase of thermal mobility of chain repeating units with the molar ratio of hexylstyrene (in the first series) or with the alkyl chain length (in the second series) manifests itself as a shift to lower temperatures of the glass transition temperature, as well as an increase of heat capacity, thermal diffusivity and effective number of external degrees of freedom per chain repeating unit from the Simha–Somcynsky reduced equation of state. The inadequacy of the simple diad additivity scheme for a quantitative description of structure–property relationships for the copolymers studied is