A combination of dielectric and thermomechanical techniques is used for studying relaxation transitions in plasticized poly(vinyl chloride) compounds at different stages of the calendering process. An unplasticized aggregate phase is discovered to be present at a fairly high concentration, even in completely processed samples. By analysing the dielectric depolarization spectra in comparison with thermomechanical creep-shrinkage curves it is concluded that a relatively large part of the PVC aggregates remains untouched by the plasticizer in each phase of the calendering process, the compound being thus a heterogeneous mixture of the aggregate and plasticized phases. This conclusion is supported by computer simulations of the Maxwell-Wagner-Sillars type polarization in this heterogeneous mixture. The dielectric depolarization method was found useful for characterizing the efficiency of processing.