Extensive mud volcanism on the Andaman accretionary prism occurs above a complex network of faults and is caused by the convergence of the Indian plate and the Burmese microplate. The mud matrix ejected from these mud volcanoes is composed of smectite–illite–kaolinite–chlorite–plagioclase–quartz–calcite assemblage derived both from sediments and altered oceanic crust and originates from a deep-burial diagenetic environment. The modes of δ13C distributions for methane (>−42‰), ethane (>−27‰) and CO2 (b−3‰) gases emitted by these mud volcanoes indicate their thermogenic origin, with TOC and N of associated mud suggesting marine organic matter as the source. The mud water is much fresher (Cl− = 45 to 135 mM) than seawater and its δ18O (−0.2 to 2.6‰)and δD (−24 to −14‰) isotopic compositions fall well below the global meteoric water line. From their trace element contents and stable isotopic compositions it is inferred that the mud water is a mixture of sediment pore water (ancient seawater) and water released from dehydration of clay minerals. The 87Sr/86Sr of mud water (~0.7071) confirms the above inference and points out that altered oceanic crust plays a significant role in controlling the chemistry of water. The formation temperatures of mud ejecta, derived from mineralogical (smectite/illite), chemical (K+/Na+) and isotopic (δD/δ18O) geothermometers, lie in the range of 50 °C to 120 °C — which corresponds to a depth zone of 2 to 6 km within the Andaman forearc. It is believed that the mud volcano ejecta originate at the plate-boundary décollement zone, from the sediments and altered oceanic crust of the subducting Indian plate.