||The Los Humeros geothermal field, situated 280 km east of Mexico City, has a large resource potential. At present, with 45 deep drilled wells that encountered more than 300oC steam, it has an installed electrical generating capacity of 96 MWe. Some wells exhibit low permeability and others produce aggressive corrosive acid fluids that are not suitable for production. However, in general, the reservoir rocks lack evidence of acid alteration. Instead, the observed assemblages of alteration minerals are typical of water-rock reactions in an alkali neutral environment. Acid alteration of rocks in volcanic geothermal fields has usually been attributed to emanation of acid gases from deeper volcanic molten rocks. At Los Humeros evidence for this is absent. Instead we suggest that the formation of the acid could be due to some post-exploitation process (e.g. reaction of water and salts forming hydrogen chloride by hydrolysis at high temperatures). Initially we studied the lithology, hydrothermal mineralogy, acid alteration and permeability of drill cores and cuttings from well H-26; which is a low permeability well drilled in a hot part of the field, with a bottom hole temperature of 378oC. Its lithological sequence is similar to that observed in most wells drilled in the field, consisting primarily of hydrothermally-altered andesite rock. The observed hydrothermal alteration mineralogy is characteristic of reactions with neutral to basic fluids. The reservoir rocks have low permeability, but locally show dissolution of hydrothermal minerals and silicification of the rocks. Dissolution has produced cavities and empty micro fractures that modify physical and mechanical properties of the rocks. In addition, in this well a zone of intense silification, about 100 m thick, occurs at about 2000 m depth, which is evidence of a lateral flow of low pH fluids. The low permeability silicified zone is composed mainly of microcrystalline quartz, with sparse plagioclase phenocrysts and traces of chlorite. It appears that this is produced primarily by leaching of other components, leaving silica behind rather than by addition of SiO2. This leaching process affecting the reservoir rocks had not been recognized by previous workers at the Los Humeros. The local silicification may be considered as the only evidence of the presence of acid fluids at depth. The high temperatures observed in the well H-26 and the nature of the reservoir rocks, together with the existence of steep conductive thermal gradients; indicate the presence of a young intrusive body close to the silicified zone. We are studying other selected wells inorder to understand the processes that have affected the deep reservoir and hence suggest possible strategies to improve the future development of this large geothermal resource.