According to the classical nucleation theory, the free energy barrier for bubble nucleation, and thereby the nucleation rate, are functions of the initial bubble pressure, P-bubble,P-0. In almost all of the previous studies that have used computer simulations to investigate polymeric foaming processes, the value of P-bubble,P-0 has been approximated using the saturation pressure, P-sat. This article employs the thermodynamic equilibrium condition and the Sanchez-Lacombe (SL) equation of state (EOS) to determine the value Of P-bubble,P-0. It is shown that using P-sat to approximate P-bubble,P-0 may lead to significant overestimations of the nucleation rate and the final cell density. (c) 2007 Wiley Periodicals, Inc.