ECO2N is a fluid property module for the TOUGH2 simulator (Version 2.0) that was designed for applications to geologic sequestration of CO{sub 2} in saline aquifers. It includes a comprehensive description of the thermodynamics and thermophysical properties of H{sub 2}O-NaCl-CO{sub 2} mixtures, that reproduces fluid properties largely within experimental error for the temperature, pressure and salinity conditions of interest (10 C {le} T {le} 110 C; P {le} 476 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 CO{sub 2}-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. This report gives technical specifications of ECO2N and includes instructions for preparing input data. Code applications are illustrated by means of several sample problems, including problems that had been previously investigated in a code intercomparison study.