An equation of state (EOS) has been developed to model thermodynamic properties of pure species and mixtures from ambient to supercritical conditions. It has been developed for use in modeling supercritical water oxidation (SCWO) of liquid and slurried organic wastes. Kinetic and flow simulations of the SCWO process require accurate predictions of densities (errors +/- 10% or less) and other thermodynamic properties from ambient to supercritical conditions of water (25 degrees C < T less than or equal to 600 degrees C; 1 bar < P less than or equal to 300 bar). Over these temperature and pressure ranges, EOSs proposed by other investigators have been unsuccessful in estimating accurate properties such as fluid densities, vapor pressures, residual enthalpies and residual entropies for water and aqueous mixtures containing carbon dioxide, nitrogen, organics and oxygen. Some improvement has been achieved using volume translation methods with cubic equations of state, but even these EOSs have limited accuracy for predicting densities. The proposed pure-component EOS couples a volume translation to a pressure-explicit equation in volume and temperature that combines a Carnahan-Starling hard-sphere repulsive term b and a simple van der Waals attraction term a. The translation constant t is determined by a fit to liquid and vapor coexistence density data while a and b are determined from critical point data. The focus of this paper is on the analysis of pure components for which the proposed EOS is shown to fit a number of important thermodynamic properties to within average deviations of 1-30% over a wide range of conditions for ammonia, carbon dioxide, ethylene, methane, nitrogen, oxygen and water.