An exact expression is reported for an association based equation of state that allows the formation of monomers, dimers and trimers. To demonstrate the utility of this exact solution, the phase equilibrium results of acetic acid are obtained from this exact expression and are compared with a numerical approach as well as a fitted functional form. The exact form is preferred over the numerical as well as the fitted function form based on issues that arise from the computational time (the former) and accuracy of derivative properties (the latter). (c) 2007 Elsevier B.V. All rights reserved.