This document presents a study into the working fluids that can be used at an Enhanced Geothermal System (EGS) plant as a way of making efficient, large-scale use of the enormous resources offered by geothermal energy. First, we investigate the two working fluids most used in such plants: water (H2O) and carbon dioxide (CO2). The comparative analysis brings to light the advantages of each one, making it possible to assess their beneficial properties. This leads us to establish what properties any alternative working fluid should have. Then, we analyse fluids included in the database of the Engineering Equation Solver (EES) program. This entails a study of their thermodynamic properties in the working conditions established. Based on the properties of each alternative working fluid and the results obtained from the EES, we seek to determine which working fluid has the best performance. Finally, the results obtained after the analysis leads us to conclude that single supercritical phase nitrous oxide seems to be an alternative to the two working fluids used to date. (C) 2018 Elsevier Ltd. All rights reserved.