In the simplified hole theory equation of state proposed by our group (Zhong, C.; Wang, W.; Lu, H. Fluid Phase Equilib. 1993, 86, 137), the occupied site fraction for the Simha-Somcynsky equation of state (Simha, R.; Somcynsky, T. Macromolecules 1969, 2, 342) was derived as a simple exponential function of temperature alone, in terms of the theory for all imperfect crystal with thermal defects. In this work, the simplified hole theory equation of state has been applied to an accurate description of the pressure-volume-temperature relationship for 67 liquid polymers and 61 solvents and compared with four commonly used equations of state. An approximate expression for the activity coefficient of a solvent in polymer-solvent solutions has been derived, which enables one to calculate the activity coefficient for the solvent analytically. The grand absolute average deviation between the calculated and experimental solvent activity coefficients for 95 data sets is 2.4%, which indicates that this equation of state is also capable of accurately describing polymer-solvent solutions.