Chemical oxygen demand is one of the important hydrochemical parameters, which characterize the living environment of hydrobionts, indicating indirectly the level of water pollution with organic compounds. Water permanganate chemical oxygen demand or permanganate index (CODMn) shows the content of easily degradable organic compounds and of some inorganic compounds (NO2 -, SO3 2-, Fe2+, etc.), and the water dichromate chemical oxygen demand (CODCr) is used for the assessment of the state of natural waters, which are highly influenced by human factor. The water sampling has been carried out during vegetation season of 2015 in the Middle Dniester (Naslavcea, Volcinet, Soroca, Camenca), Dubasari reservoir (Erjova, Goieni, Cocieri) and the Lower Dniester (Vadul lui Vodă, Varnita, Sucleia, Palanca). For determination of CODMn the standard SM SR EN ISO 8467:2006 was used, of CODCr - SM SR ISO 6060:2006. On the base of obtained data, the organic matter content was calculated by using the method of Aleokin (1973). It was revealed that in 2015 the values of CODMn ranged from 4.89 to 13.81 mgO/l, those of CODCr varied from 23.64 to 100.92 mgO/l. In the case of both parameters the highest figures were registered in summer and autumn at Soroca station. The ratio between CODMn and CODMn, which fitted within diapason 0.12-0.31, proved a low intensity of self-cleaning processes in the river waters. According to the Moldovan Regulation on environment quality requirements for the surface waters (2013), the values of CODMn indicated good - moderately polluted waters (classes II and III of water quality), but those of CODCr – even polluted and very polluted waters (classes IV and V) in the Dniester River. During the study the highest content of organic matter was found in summer at Soroca – 75.7mg/l, which is determined by the heavy pollution of natural waters primarily with household waste waters. Altogether for 2015, in 72.7% of the investigated samples in Dniester river the organic matter exceeded 20 mg/l. In 2013-2014 the content of organic matter was in average of 21.2 mg/l, when in 2015 – 26.6 mg/l, this fact being explained mainly by the lower water level in the last year on Moldovan territory. To note its content increased from spring to autumn. Acknowledgement. The work was performed in the frame of the institutional project 15.817.02.27A of the Academy of Sciences of Moldova.