The group-contribution lattice-fluid equation of state (GCLF-EOS), which is capable of predicting equilibrium behavior in polymer systems, was developed by establishing group contributions of the lattice-fluid EOS using the PVT properties of low molecular weight compounds only. This model was used to predict the PVT behavior of common polymers over a wide temperature range in the melt region and over a wide range of pressures up to about 2,000 bar. The GCLF-EOS predicted accurately the effect of pressure and temperature on the specific volumes of the polymer melts. Prediction results by the GCLF-EOS were compared with those by the group-contribution volume (GCVOL) method. The GCLF-EOS requires only the structure of the polymer repeat unit in terms of their functional groups as input information. No other polymer properties are needed. The GCLF-EOS is the only model that is capable of predicting the specific volumes of polymer melts as a function of temperature and pressure.