In 2010, 2011, and 2013 we assessed the water of the Čičov Oxbow National Nature Reserve in the southwestern part of the Slovak Republic for concentrations of organic substances. Collections of samples were realized regularly at monthly intervals, always about in the half of the month. Sampling sites were determined in order to assess the impact of natural and anthropogenic source of surface water quality. The results were compared from the limit values set out in Government Regulation No. 269/2010 Coll. The average value of BOD₅ for the whole period was 1.58 mg O₂ dm^-3. The highest average concentration of biodegradable organic matter was in February and the minimum in May. Depending on the sampling site, we found the highest average value for the whole monitored period in the northeastern part of Čičov Reserve, and the lowest in the sampling site situated approximately 150 m from the mouth of Čilizian Stream in the reserve. The average value of COD_Cr for the entire monitored period represented 60.69 mg O₂ dm^-3. Depending on the time of sampling, minimum average value was in February and maximum in September. Depending on the sampling site, we found the highest average value for the whole monitored period in the sampling site located approximately 150 m from the mouth of the Čilizian. The average value of the BOD₅/COD_Cr ratio during the years 2010, 2011, and 2013 was 0.026.