Shallow submarine hydrothermal activity has been observed in Western Mexico related to extensional tectonic faults. Hydrothermal. vents occur at Punta Banda on the Pacific coast of Baja California, at Bahia Concepcion on the eastern coast of Baja California, and in Punta Mita on the Pacific coast of central Mexico. Submarine discharge of geothermal fluids is located at depths varying between 5 and 30 in. Water and gas discharge at temperatures between 40 and more than 100 degreesC. The composition of the thermal end-member can be calculated for Mg = 0 using the chemistry data of the water samples. A linear regression of the concentration values vs magnesium content is used to determine the concentration of the end-member thermal water. The chemical composition of the thermal end-member indicates that the water is more dilute than seawater and enriched in Ca, Mn, Ba, I, Cs, B, Li, Rb, Sr and Si. The results show that the water chemistry is similar in these coastal hydrothermal systems. The thermal water is probably of meteoric origin, penetrating through the extensional faults, and heated by the high geothermal gradient. The components in the thermal water are contributed by the deep strata. (C) 2003 CNR. Published by Elsevier Ltd. All rights reserved.