ESR has been used to observe the paramagnetism of a poly-conjugated system (PCS) induced by heat, i.e. of modified polyvinyl chloride (MPVC) at low temperatures (77–340°K). The energy of the triplet level of the PCS in MPVC (about 0·02 eV) was estimated. The paramagnetism of MPVC between 340 and 430°K is due to a transition of the polyenes from the triplet state into stable biradicals. The paramagnetic properties of the reaction product of the polyene system present in MPVC with the halides of a PVC modified by thermal radiation have been analysed. There is an activational dependence of the ESR signal intensity, typical for strong complexes with a charge transfer and thermal excitation of the paramagnetism. The singlet-triplet transition in the complexes has been found to have an energy of 0·025 –0·04 eV, depending on the halide type present.