Many research and development projects throughout the world are devoted to sustainable hydrogen production processes. Low-temperature electrolysis, when Consuming electricity produced Without greenhouse gas emissions, is a sustainable process, though having limited efficiency. The performance of electrolysis processes can be improved by functioning at high temperature (high-temperature electrolysis, HTE). This leads to a reduction in energy consumption but requires some of the energy necessary for the dissociation of water to be in the form of thermal energy, and the ability to recover some of the heat contained in the outlet products of the electrolysis. Heat would be obtained by coupling the process either to a high-temperature reactor (HTR) or to a geothermal source. A techno-economic optimisation of the upper heat exchanger network in the HTE process was conducted. We chose the method of optimisation using genetic algorithms. As expected, the optinused heat exchanger networks appear to differ depending on the heat source that is used. Partly due to very different unit costs for thermal energy, the contributions of the investment costs are really different. We may also note that the cost of the input heat is increased by 10% when Coupling the system to a HTR, whereas it is more than doubled in the geothermal case. (c) 2006 Elsevier Ltd. All rights reserved.