Biological oxygen demand (BOD) and chemical oxygen demand (COD) are globally used as quality parameters for the evaluation of organic pollution in water environment. COD and BOD5 analyzes were carried out in accordance with national standard methods in water samples collected in 2020 from the Prut River. CODMn values have not varied essentially from site to site during the winter and spring periods and ranged from 5.8 – 7.4 mg/l and 4.6 – 4.9 mg/l, respectively. High CODMn values in winter, comparing to those from spring, were due to the low water level in the river. The CODCr values showed a clear increase in winter along the Costesti – Giurgiulesti river section from 18.9 to 45 (mg/l). During the low water period high COD values could be provoked by the discharge of insufficient treated wastewaters. In spring time the higher water level in the Prut River contributed to dilution effect, and the COD values were quite stable on the studied river segment. The BOD5/CODCr relation is often used to assess the efficiency of biological treatment for industrial wastewater: if the index value is > 0.2, biological treatment can be effective. On the other hand, the BOD5/CODCr index calculated for a water body allows evaluating the capacity of self-purification at different sections of the river. A higher value of the index characterizes a greater ability to self-purification of water. A comparative analysis of the BOD5/CODCr index has been done based on the results of a winter and spring expeditions at the Prut River. To mention that the spring expedition was carried out at the end of May, when the rain flood began on the Prut River. As for rainwater, the BOD5/CODCr index is very low. The index values in spring samples of the Prut River, containing a significant proportion of rainwater, were also low and ranged from 0.052 to 0.068. Index values in winter samples varied significantly: from 0.05 at Braniste station to 0.16 and 0.18 at Cislita and Leuseni stations, respectively. Thus, the biological purification process can be most effective at the Leuseni and Cislita stations. Conclusion: the proportion of the organic component in the substances, which are oxidized during the determination of CODCr, varies significantly depending on the hydrological situation on the Prut River. At the same time, applying the BOD5/CODCr index helps to identify points of risk for reaching a good ecological state.