With a simple thermodynamic analysis, we show that polymers do not satisfy the two major criteria that are required for establishing a unique relationship between cohesion energy density (CED) and internal pressure (IP). Since IP can take values independently of CED, we contend that the IP/CED ratio cannot be established a priori, as the ratio depends on the thermodynamic variables (temperature, pressure, etc.) and the system of interest. (c) 2005 Society of Plastics Engineers.