A single-phase equilibrium reactor model has been developed, that calculates the reaction equilibrium at a fixed pressure by minimization of the Gibbs free energy at constant temperature, or a maximization of entropy at constant enthalpy. An automatic procedure of determining the reactions and stoichiometry is applied. The reactor model has been implemented in compliance with the CAPE-OPEN standards, and can therefore run in multiple simulation environments using different thermodynamic engines. Care must be taken by the unit operation not to evaluate thermodynamic properties at conditions for which the thermodynamic server may not provide answers. Specifically, it is important to evaluate thermodynamic properties only at mole fractions in the [0,1] region. A projection algorithm has been applied to ensure this. The reactor model has been tested in different simulation environments and is available in the free-of-charge COCO simulator suite. (c) 2010 Elsevier Ltd. All rights reserved.