The relevance of the work consists in the need to study the water quality of the Siverskyi Donets River and its tributaries at the beginning of the 21th century, since the river is a main source of water supply in the eastern part of Ukraine. The catchment area of the Siverskyi Donets River is located in the most industrialized region of Ukraine. It is a transboundary river and its catchment is located in the territory of both Ukraine and Russia. The volume of the river's water use, if compared to its runoff volume, is the largest in Ukraine. The condition of water quality is determined by high water consumption and large amount of polluted water being discharged into it. The largest impact is caused by discharges from the residential and commercial complex (big cities) and the enterprises of coal industry, ferrous metallurgy, chemical and petrochemical industry which are concentrated in the Donets Coal Basin. The purpose of this work is to assess the changes of surface water quality of the Siverskyi Donets Basin and to establish the role of the tributaries affected by considerable anthropogenic pressure in the formation of the ecological state of the main river. The research aims at analyzing the hydrochemical indices of the Siverskyi Donets River and its tributaries: the Udy River (9 km to the south from Kharkiv) located in the forest-steppe zone and affected by the wastewater of Kharkiv megalopolis; the Oskil River (Chervonooskilske Reservoir) located in the karst area, having a significant underground water supply and the runoff of which is regulated by the reservoir; the Luhan, Kryvyi Torets, Kazennyi Torets, Bakhmut rivers that are tributaries originating from the Donetsk Highlands and affected by the wastewater of Donbas industrial and municipal enterprises. The subject of the research is the ecological condition of water resources in the rivers of the Siverskyi Donets Basin estimated using the generalized ecological index. The water quality condition was analyzed based on the hydrochemical observations performed from 1990 to 2015 by Central Geophysical Observatory in Kyiv. The main method of research is the method of ecological assessment of surface water quality of land and estuaries in Ukraine using relevant categories. The assessment of the surface water quality using the generalized ecological index (with application of average indicators) showed that the water quality deteriorates along the main river with its worst condition at Lysychansk section. Analysis of the tributaries indicated the worst ecological condition of the Bakhmut River (Donbas). For most of the considered rivers the 3rd class of quality (“satisfactory”), the 4th category ("satisfactory condition and slightly contaminated”) were established. If we consider the index of ecological condition by maximum indicators, the results are almost the same for all the studied rivers: 5th class, 7th category ("very poor condition and very contaminated”). According to the block of salt composition components, the worst ecological condition is established for sections of the Siverskyi Donets River (city of Lysychansk) and the Bakhmut River. The analysis of the generalized ecological index dynamics along the length of the main river indicated a general trend (except for the section in Lysychansk) to water quality improvement. This was achieved following the decrease of toxic effects due to reduction of petroleum products and phenol concentrations. No significant changes in the index were detected for the tributaries. Donbas rivers are also characterized by increase of the salt composition components index: their water is classified as "brackish" whereas the water of other tributaries is classified as “fresh water”. The pollution with nitrogen compounds still remains significant and keeps increasing. The heavy metals content decreases very slowly. The inflow of water of the Donbas rivers to the main river causes significant changes of the ecological condition at the Lysychansk section located downstream from the place of their confluence.