The chemical modification of poly(vinyl chloride) (PVC) in reaction with phenol, in the absence of a catalyst and at temperatures within the range of 60 degrees C and the boiling temperature of phenol (180 degrees C), led to the reaction products with the general structure of a vinyl chloride-vinyl phenol copolymer. The synthesized polyphenols were thermally characterized using the following experimental techniques : thermo-optical analysis (TOA), thermogravimetry (TG), derivative thermogravimetry (DTG), and pyrolysis in combination with gas chromatography (P-GC). The PVC samples modified with phenol show a thermal stability lower than the original PVC sample. The glass transition temperatures (T(g)s) increase with increased degree of chlorine substitution. The main pyrolysis products of the modified PVC samples are hydrocarbons with low boiling points (C-1-C-4), benzene, toluene, naphthalene, indan, and phenol. The semiquantitative estimation of the pyrolysis products of the synthesized samples led to the conclusion that the following structural element types can be present, statistically distributed along the chain : vinyl chloride and vinyl phenol units, acetylenic and ethylenic units, and indan type structures.