Abstract. As part of the GEMex Project, an on-going European-Mexican effort to develop geothermal energy from non-conventional sources, preliminary geological models have been constructed for two sites located in the easternmost region of the Trans-Mexican Volcanic Belt. The first site, Los Humeros, which has produced geothermal electricity for decades, is investigated for its probable superhot geothermal resources. The second site, Acoculco, is a less known but promising area where application of an Enhanced Geothermal System is being studied. In order to have a coherent geological interpretation of both sites, preliminary 3-D models were constructed in a collaborative manner by European and Mexican partners. These models are based on data available at the start of the project, including geological maps, cross-sections and well logs. The data were mainly provided by the Comisión Federal de Electricidad (CFE), and the Mexican Centre for Innovation in Geothermal Energy (CeMIE-Geo consortium). A regional model was developed for each site and an additional local model was constructed for Los Humeros. The preliminary geological models serve as a framework for GEMex work on heat-transport and fluid-flow simulations; they will be updated and refined during the project, using new data and interpretations from ongoing and future field work on geology, geophysics, and geochemistry.