In El Salvador, Central America, active deformation takes the form of a major dextral strike-slip fault system, the El Salvador Fault Zone, resulting from the oblique subduction of the Cocos Plate. The fault system is laterally discontinuous, being subdivided into different major en-echelon segments that partially overlap to form pull-apart structures. Volcanic activity is spatially confined to the fault segments and absent in the intervening pull-apart basins; no significant temporal gap exists in the erupted products, at least during the Plio-Quaternary. Detailed analyses within the geothermal fields of Berlin and Ahuachapan have revealed important volcano-structural and petrologic differences between the two areas. In the Berlin area active deformation is controlled by the regional transcurrent stress field, resulting in the development of systems of right-lateral E-W-trending strike-slip faults. Conversely, the structural setting of the Ahuachapan area is more complex, reflecting an interaction among different stress fields. Berlin products exhibit a marked geochemical and isotopic homogeneity indicating the presence of a single magmatic system. At Ahuachapan, on the other hand, the rocks display significant variations in both Sr isotopes and the LILE/HFSE ratios: this area is characterized by multiple volcanic centres, fed by different magma batches that reach the surface without reciprocal interactions in shallow reservoirs. Thus, the characteristics of the volcanic products at Berlin and Ahuachapan reflect their different tectonic settings, with important implications for geothermal investigations. (c) 2006 Published by Elsevier Ltd on behalf of CNR.