The relevance of the study is caused by the lack of effective indicators to date that allow classifying fluid systems in accordance with their substance composition, geological settings and dynamics of fluids discharge to the earth's surface. Studies of the chemical and isotopic composition of the water from fluid systems will make it possible to determine the commonality and differences in origin of fluid systems of different types, as well as to describe the main geochemical processes occurring in them. This is necessary for theoretical understanding of the mechanisms and patterns of activity of different fluid systems. Problems of hydrogeochemical classification are also of great practical importance, since different fluid systems imply various scenarios for development of hazardous geological processes associated with them. Apart from that, some fluid systems are associated with the generation and migration of hydrocarbons, which can be used as one of the prospecting signs in exploration. The aim of the study is to determine possible indicators for recognizing fluid systems of different types using hydrogeochemical characteristics. Objects: waters from different systems of focused discharge of subsurface fluids – Hydrocarbon Sedimentary (mud volcanoes and thermomineral springs), Sediment-Hosted Hydrothermal (geothermal seepage of the Salton Sea and Lusi) and Volcano & Hydrothermal Geothermal (mud pools and hydrotherms on magmatic volcanoes). Methods. Concentrations of the main anions (Cl^–, Br^–, SO₄^2–) and cations (Li⁺, Na⁺, K⁺, Ca²⁺, Mg²⁺) in the studied waters were determined by the method of ion chromatography; HCO₃^– concentration was determined by the titrimetric method; the boron content was determined by the spectrophotometric method. The isotopic composition of waters (δ¹⁸O, δD) was determined by infrared laser spectrometry. Subsequent processing of hydrogeochemical data was done using the methods of mathematical statistics. The comparative study of waters from fluid systems of different types was conducted using the generally accepted geochemical diagrams. Results. The present paper is a comparative study of a wide range of hydrogeochemical indicators of fluid systems – Hydrocarbon Sedimentary, Sediment-Hosted Hydrothermal, and Volcano & Hydrothermal Geothermal. It was found that these indicators for the studied groundwater have a wide range of values. Consequently, for some fluid systems there is no clearly pronounced differentiation according to the analyzed physico-chemical parameters. In particular, no significant differences were found in chemical composition of the waters from mud volcanoes and the Sediment-Hosted Hydrothermal System Lusi. It does not allow solving the problem of recognition of these fluid systems using only hydrogeochemical characteristics, or casts doubt on the correctness of antecedent statements about their different origin. At the same time, Volcano & Hydrothermal Geothermal systems are characterized by unique hydrogeochemical properties that differ significantly from other types of fluid systems. The most informative signs for these fluid systems are the pH, which mainly varies from ultra acidic to acidic values, and the geochemical type of waters, characterized by a mixed cationic composition with a high content of aluminum and iron. In addition, the waters of most of the studied fluid systems are characterized by high content of boron and lithium, but their concentration ranges often overlap. At the same time, the sources of these chemical elements for the fluid systems of the regions of modern magmatic volcanism differ from the fluid systems associated with sedimentary basins. The results obtained indicate the need to revise the existing geochemical classifications of fluid systems.