The Middle Ob is the part of the Ob river reach running through the central part of Western Siberia within Khanty-Mansi Autonomous Okrug, and brings its waters through the territory of Yamalo-Nenets Autonomous Okrug to the Gulf of Ob of the Arctic Ocean. The data on hydrochemical characteristics of the Middle Ob river and its anabranches during 2003-2008 and 2014-2018 in aspects of transboundary transfer is presented in the article. Composition of salt, organic compounds, heavy metals, petroleum hydrocarbons, biogenic ions in water and bottom sediments are described. The influence of oilfields and urban territories in water catchment area of the Middle Priobie is revealed. The main pollutants of the Middle Ob river are phenols, hydrocarbons, iron, manganese, chrome, cooper, phosphate-ions from technogenic and natural originating. We investigated that the level of the Middle Ob river water pollution was increased from 2003-2008 till 2014-2018.