ECO2N is a fluid property module for the TOUGH2 simulator (Version 2.0) that was designed for applications involving geologic storage of CO2 in saline aquifers. It includes a comprehensive description of the thermodynamics and thermophysical properties of H2O-NaCl-CO2 mixtures, that reproduces fluid properties largely within experimental error for the temperature, pressure and salinity conditions of interest (10 degrees C <= T <= 110 degrees C; P <= 600 bar; salinity up to full halite saturation). Flow processes can be modeled isothermally or non-isothermally, and phase conditions represented may include a single (aqueous or CO2-rich) phase, as well as two-phase mixtures. Fluid phases may appear or disappear in the course of a simulation, and solid salt may precipitate or dissolve. ECO2N can model super-as well as sub-critical conditions, but it does not make a distinction between liquid and gaseous CO2 and hence is not applicable for processes that involve two CO2-rich phases. This paper highlights significant features of ECO2N, and presents illustrative applications. (C) 2007 Elsevier Ltd. All rights reserved.