SummaryECO2N V2.0 is a fluid property module for the TOUGH2 simulator (Version 2.1) that was designed for applications to geologic sequestration of CO 2 in saline aquifers and enhanced geothermal reservoirs. ECO2N V2.0 is an enhanced version of the previous ECO2N V1.0 module (Pruess, 2005). It expands the temperature range up to about 300 o C whereas V1.0 can only be used for temperatures below about 110 o C. V2.0 includes a comprehensive description of the thermodynamics and thermophysical properties of H 2 O -NaCl -CO 2 mixtures, that reproduces fluid properties largely within experimental error for the temperature, pressure and salinity conditions of interest (10 °C < T < 300 °C; P < 600 bar; salinity up to halite saturation). This includes density, viscosity, and specific enthalpy of fluid phases as functions of temperature, pressure, and composition, as well as partitioning of mass components H 2 O, NaCl and CO 2 among the different phases. In particular, V2.0 accounts for the effects of water on the thermophysical properties of the CO 2 -rich phase, which was ignored in V1.0, using a model consistent with the solubility models developed by Pruess (2005, 2010). In terms of solubility models, This report gives technical specifications of ECO2N V2.0 and includes instructions for preparing input data.