An equation of state for zero internal pressure in rare gas solids and semi-crystalline polymers has been determined based on the empirical functions of thermal pressure coefficient gamma(V) with respect to volume at constant pressure. The experimental data of PVT over wide range of temperature and pressure published by Anderson and Swenson and Syassen and Holzapfel for rare gas solids and Olabisi and Simha and Zoller for semi-crystalline polymers are used to evaluate gamma(V). The function of gamma(V) with respect to volume determined at constant pressure is given by gamma(V) = A{(V - V-0)/V}(epsilon)(V-0/V)(1+c) exp{-c(V - V-0)/V} where V-0 is the volume at 0 K, A, epsilon and c are constants. The function of internal pressure P-i = gamma T-V - P with respect to temperature at constant pressure is determined by converting the function of gamma(V)(V) to a function of temperature gamma(V)(T). An empirical equation of state for zero internal pressure determined by pressure P*, volume V* and temperature T* at which P-i = 0 is expressed by P*V*/RT* - C* - D*V* for rare gas and semi-crystalline polymer where C* and D* are constants. The practical meaning of the equation of state for Pi = 0 in the semi-crystalline polymers has been discussed. (c) 2006 Elsevier Ltd. All rights reserved.