The thermodynamic property package estimates phase equilibrium using modified Raoult's law with activity coefficients estimated through Dortmund UNIFAC interaction parameters. Modified Raoult's law is suitable to estimate phase equilibria of nonideal mixtures at low to moderate molecular weights and pressures up to 10 atm. Therefore, this phase equilibrium model can accurately predict the interactions of the highly polar, low molecular weight compounds present in the biorefinery (e.g., water, methanol, glycerol). The lignocellulosic components are modeled following the pure component property values used by NREL (heat of formation, molecular weight, etc.), with the exception that liquid densities and heat capacities for lignocellulosic components are held constant at 25 ⸰ C. Pure component properties (heat capacity, density, etc.) of fluids are estimated using higher order polynomial fits to the fundamental Helmholtz equation of state (a state-of-the-art property prediction model), recommended correlations from critical reviews on thermodynamic properties, and equations from well-established public references such as the CRC handbook of chemistry and physics and the NIST Webbook. Mixture properties are estimated using a molar weighted average of the pure chemical properties. This property package has been validated against a cellulosic biorefinery model by NREL implemented in Aspen Plus and both sugarcane and oilcane biorefinery models in SuperPro. 11