The dielectric relaxation characteristics of polyamide-12 (polydodecalactam), plasticized by sulphonamides of the type RSO 2NHC 4H 9 (where R is phenyl, β-naphthyl, 3-pyridyl or 8-quinolyl), were studied over a wide range of temperatures and frequencies (−100°C to + 100°C and 0.5–200 kHz). The glass temperature and relaxation time were determined, and the free energy, enthalpy and entropy of orientatational dipole polarization of the systems were calculated. It is shown that the decrease in all these characteristics as the sulphonamide concentration is increased, indicates increase in the mobility of the structural elements of the system, and that heterocyclic and bicyclic sulphonamides give an improved plasticizing effect.