An association model has been incorporated in the Peng-Robinson equation of state (PREOS) to use the combined model for water/reservoir crudes phase-behavior calculations. The interaction coefficients between H2O and hydrocarbons follow a simple relationship. Such a relationship makes the use of the association equation of state (AEOS) both accurate and simple. The computed high-pressure crude/steam distillation results by the AEOS are in excellent agreement with the data.