In a former paper the authors suggested the following equation of state, (P+π)(V-ω)=RT, where π is the internal pressure (or cohesive energy density) and ω is the volume at absolute zero. It was found that this equation was in good agreement with data on the thermal expansivity and isothermal compressibility of polystyrene. The present paper reports isothermal compression measurements on polymethyl methacrylate, polyethylene, cellulose acetate‐butyrate, and ethyl cellulose at temperatures within the fabrication range. The above equation of state is shown to apply equally as well to these thermoplastics as to polystyrene and values are given for the constants. The usefulness of this relationship is illustrated in a brief discussion of some phases of the injection molding process.