Daugava water treatment is the largest plant in Latvia supplying about 50% of the drinking water for Riga city (100.000 m3 daily). The plant takes water from River Daugava (from the reservoir of Riga hydroelectric power station). The watershed of the Riwer Daugava is largely covered with swamps (7%) and forests (38%); therefore, water in the river contains a high amount of humic substances (Rubulis and Juhna, 2005). Upstream the intake several villages and cities are located (in Russia, Belarusia and Latvia) therefore the raw water is influenced by wastewater discharges. To reduce the pollution risk of the drinking water in 2001 the treatment process was upgraded with two stage ozonation. Consequently, Daugava WTP is using preozonation, chemical coagulation with alum, rapid filtration, main ozonation, biologically active carbon (BAC) filtration and final chlorination. Here we present results of monitoring of natural organic matter (NOM) removal at Daugava Water Treatment Plant (WTP) and river Daugava. The aim of this study was to determine the fraction and types of compounds of NOM produced and removed by the treatment processes, in particular during ozonation and biofiltration. NOM changes were investigated by BDOC measurements (according to Ribas et al., 1991), NOM-fractionation (according to Christopher et al., 2004), Fourier Transform Infrared (FT-IR) spectrometry and Liquid Chromatography-Organic Carbon Detection (LC-OCD). This was undertaken in order to gain more information for optimization of NOM removal efficacy at the plant. Total organic carbon (TOC) measured in river Daugava was 12.4 mg/l in 2006 and increased with 50% in 2008 reaching 18.7 mg/l. Dissolved organic carbon (DOC) contained 95% of TOC. During the treatment process in Daugava WTP the DOC concentration decreased with 68% and average result in the drinking water was 4.99±1.92 mg-C/L in study period. Results of the analyses of water samples showed that preozonation-coagulation and filtration reduced the concentration of very hydrophobic acid fraction (VHA; humic substances/high molecular weight) whereas other fractions (biodegradable dissolved organic matter; low molecular weight acids (LMA)/hydrophilic charged acids) remained almost unchanged during the treatment process.