A geochemical study was carried out on gas, water and mud samples from four mud volcanoes in Gorgan Plain, SE Caspian Sea (Iran) in order to investigate fluid primary sources and secondary processes controlling fluid chemistry. The chemical composition of light alkanes and the isotopic feature of methane indicated an origin related to a thermogenic source. Gases discharged from Neftlijeh evidenced anaerobic biodegradation processes with addition of secondary microbial methane. Chemical composition of discharged waters revealed two main groups i) brine-type Na⁺–Cl⁻ waters from Gharenyaregh and Neftlijeh mud volcanoes, which were marked by relatively high Na⁺/Cl⁻, B/Cl⁻ and Li/Cl⁻ ratios and low Ca²⁺/Cl⁻, Mg²⁺/Cl⁻ and K⁺/Cl⁻ ratios, ii) waters from Sofikam and Inche, characterized by relatively low Na⁺/Cl⁻, B/Cl⁻ and Li/Cl⁻ ratios and relatively high Ca²⁺/Cl⁻, Mg²⁺/Cl⁻ and K⁺/Cl⁻ ratios. The chemical and isotopic characteristics of the discharged waters suggest that evaporated Caspian seawater trapped in sediments is likely representing a reliable water source. The maximum formation depth at Gharenyaregh and Neftlijeh mud volcanoes were estimated at about 6 km depth whereas the generation depth of the rest was significantly shallower. Thus, the observed compositional differences can be related to the different depths of the fluid source feeding them.