The volumetric properties of five random copolymers have been studied, both experimentally and theoretically, over extended ranges of external conditions of temperature (up to 500 K) and pressure (up to 200 MPa). The experimental measurements have been conducted on a high pressure densitometer (SWO, model PVT100). In copolymers of the type vinyl chloride-co-vinyl acetate, simple linear combining rules are sufficient to describe properly the PVT behavior by using an equation of state appropriate for copolymers, This, however, is not true in copolymers of the styrene-co-acrylonitrile type where the repeating units are significantly different in size and shape. The combination of our theoretical and experimental approach permitted the assignment of equation of state scaling constants to the fictitious polymer poly(maleic anhydride). This is of importance since maleic anhydride is one of the most widely used repeating units in polymer alloying and blending, The proposed equation of state model also permits the study of systems of the type styrene-co-vinyl phenol, in which a highly complex network of hydrogen bonding is involved.