Abstract: A new equation of state based on the two - fluid coordination model for polymer solutions was applied to correlate the experimental pVT data of three copolymer melts [P(VAC/E), P(S/MMA) and (PEP)] and two blend systems (PVME/PS and PPO/PS) with high accuracy, and a new assumption considering a copolymer melt as a blend system was proposed to define the segment length of random copolymer. By correlation of the pVT data of the copolymer melts and the blends, it was found that thecross energetic parameter η12 and average hard - core volume V*12 have certain summation relationships with their corresponding homopolymer model parameters respectively. So a simple mixing rule was proposed to estimate the cross parameters to predict the pVT data. It shows that the predicted results are in good agreement with the experimental data.