The volcano Chichontepeque (San Vicente) is one of the nine recent volcanoes making up the El Salvador sector of the WNW-ESE-trending active Central American volcanic belt. Thermal activity is at present reduced to a few thermal springs and fumaroles. The most important manifestations (Agua Agria and Los Infernillos Ciegos) are boiling springs and fumaroles located on the northern slope of the volcano (850 m a.s.l.) along two radial faults. The chloride acid waters of the Los Infernillos area are partly fed by a deep hydrothermal aquifer (crossed at 1100-1300 m by a geothermal exploration well), which finds a preferential path to the surface through the radial fault system. CO2 is the most important gas (>90%) of the Los Infernillos Ciegos and Agua Agria fumaroles. Part of the Los Infernillos gases may also come from a deeper, hotter source, given their high HCl/S-tot. ratio and their more reducing conditions. The application of geothermometric and geobarometric methods to the gases and thermal waters suggests that both thermal areas are linked to the identified 1100-1300 m reservoir, whose temperature (250 degrees C), lateral extension and chemical composition, as resulting from this study, are of interest for industrial development.