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Shallow submarine hydrothermal activity has been observed in the Bahía Concepción bay, located at the Gulf coast of the Baja California Peninsula, along faults probably related to the extensional tectonics of the Gulf of California region. Diffuse and focused venting of hydrothermal water and gas occurs in the intertidal and shallow subtidal areas down to 15 m along a NW–SE-trending onshore–offshore fault. Temperatures in the fluid discharge area vary from 50 °C at the sea bottom up to 87 °C at a depth of 10 cm in the sediments. Chemical analyses revealed that thermal water is enriched in Ca, As, Hg, Mn, Ba, HCO₃, Li, Sr, B, I, Cs, Fe and Si, and it has lower concentrations of Cl, Na, SO₄ and Br than seawater. The chemical characteristics of the water samples indicate the occurrence of mixing between seawater and a thermal end-member. Stable isotopic oxygen and hydrogen composition of thermal samples …