Environmental and economic concerns are motivating manufacturers and public entities towards the use of renewable energy, which is continuously increasing its market penetration. Among the possible renewable energy resources, geothermal is particularly attractive compared to others such as solar and wind, mainly because of its continuity and dispatchability. Most studies in geothermal energy conversion systems model the resource as pure water or steam, while in reservoir simulations it is common practice to apply advanced geochemical modelling to estimate the long-term productivity. The presence of CO2 and saline equilibria may determine power plant optimisation conditions that differ from the ones which assume the resource as pure water or steam. The thermodynamic properties of the geothermal resource with high carbon dioxide (CO2) contents (1 to 8% in mass) within the 298 – 473 K temperature and 15 bar pressure are examined. The model applied focuses on the Equations of State (EoS) to be used in the calculation of the physical/thermodynamic properties of a mixture. The obtained results are validated through the comparison of different commercial software (UNISIM®, EES®, REFPROP®, TREND®). As an example, the Torre Alfina geothermal resource data (Middle Italy) are considered and the effects on the performance of a binary (ORC) geothermal power plant producing are examined in terms of energy and exergy efficiency. A sub-critical Iso-butane and a supercritical R134a power cycle are compared, and the advantages of the supercritical solution are demonstrated.