Plasticized PVC samples of different plasticizer concentration were tested by the thermally stimulated discharge current (TSD) method. Processing conditions were also varied. It has been found that plasticized PVC consists of at least three structural units having different molecular mobility. Their amount and characteristics strongly depend on the processing conditions, i.e. the thermal- and shear history of the material. TSD method because of its excellent resolving power proved to be a useful tool detecting these structural units.