Detailed characterization of clay minerals has been undertaken in four drill holes of the geothermal field oi Chipilapa (El Salvador) in order to improve the knowledge of clay formation and the processes of the conversion of mixed layers I-S and C-S during the geothermal activity. The high enthalpy geothermal field of Chipilapa is very recent; it is located within volcanic and volcanoclastic formations dated from Pliocene to Holocene age with a composition ranging from dacite to andesite. At the present time, the major hydrothermal activity is restricted to two highly permeable zones near a vertical fault network: a shallow vapour-dominated reservoir (500-600 m); a deeper liquid-dominated reservoir (1 100-1 400 m). The coupling of petrography with the microthermometry of fluid inclusion shows that three alteration stages successively occured and were superimposed in the geothermal field. All generated assemblages include clay minerals: (1)the first stage corresponding to thermal metamorphism (propylitic alteration), generated a chlorite + epidote + quartz +/- zeolites assemblage; (2) the later stage generated calcite + I-S + C-S + hematite assemblage. It corresponds to the epithermal alteration during which the upper part of the field was fractured, depressurized and infilled by fluids oi a very low salinity which provoked the sealing of the fracture network by carbonate deposits and crystallization of haematite and clays in adjacent wail rocks; (3) the current hydrothermal activity is interpreted as the continuation of the epithermal activity in the zones which are still permeable. The alteration assemblage is smectites + hematite + I-S (rich in smectite). The distribution of I-S and C-S mixed layers versus in-hole temperature and depth distinguishes the low permeable parts of the field from zones of current fluid infiltration. In low permeable zones, the general evolution oi I-S and C-S is characterized by a decrease in smectite with increasing depth. This evolution is rapid at a depth ranging from 400 to 600 m, where in-hole temperature increases by 70 degrees C. Over this range oi depth, the transitions I-S RO - I-S R1 (regularly ordered 35-40 %S) and I-S R1 (regularly ordered 35-40 %S)- I-S R1 (S < 15 %) are discontinuous. The transition saponite-corrensite and corrensite-C-S (S < 10 %) is also dicontinuous with persistence of triphased assemblages at depth ranging from 400 to 600 m. Below this depth, the conversion of C-S to chlorite is regular. in the permeable zones (reservoirs), the I-S and C-S distribution does not agree with a general deacrease in the expandable component with increasing depth and/or temperature. Smectites or smectite-rich mixed layers accompany poor expandable mixed layers which are similar to the mixed layers of the surrounding low permeable zones. Such a phenomenon leads to the association of two or three different types of I-S in core samples. in the reservoirs, saponite coexisting with iron-rich beidellite is common at temperatures near 200 degrees C. Near the surface area where there are low permeable rocks and low temperatures (< 130 degrees C), saponite, nontronite and di-smectite with intermediate composition between montmorillonite and beidellite coexist. The current crystallization of smectites in reservoirs is related to the local boiling oi hydrothermal fluids. The significance of these smectites is kinetic rather than thermic. Because of their temporary existance, smectites are an indicator oi the recent evolution of the geothermal activity in the field. Clay formation at high temperature favours the beidellitic composition of di-smectite. The thermal fossil gradient inferred from the distribution of I-S acid C-S in the low permeable parts of the field is not representative of a particular thermal event; it represents the integration of the whole thermal activity subsequent to the occurrence of clays. Such a reconstituted thermal gradient minimizes the effects of brief past thermal events. Transposition of information obtained from mixed layers of geothermal systems in a diagenetic series, or inversely is not correct because mixed layers differ not only by their condition of formation and transformation, but they also differ by the composition of the initial smectitic material : essentially montmorillonite in diagenesis, mainly saponite + beidellite in high enthalpy geothermal fields. These results point out the question of the nature of the smectite layers interstratified with illite within I-S R1 mixed layers which occurred at great burial depth (T > 100 degrees C). is it montmorillonite or beidellite? The answer to this question would bring about a consequent contribution to the understanding of the mechanisms oi I-S conversion.