The effect of different oxidation and filtration processes on the amount and character of organic matter in drinking water was studied. TOC, KMnO4 value, colour and fractions of humic material were analyzed from bank filtrated lake water, chemically coagulated and sand filtrated water, ozonated, O3/H2O2 treated, chlorinated, granular activated carbon (GAC) filtrated and nanofiltrated water. Size exclusion chromatography was used to separate different fractions of humic molecules. Six different fractions were found from bank filtrated lake water and from chemically coagulated and sand filtrated water. The molecular weight of humic fractions ranged from about 1000 (the last fraction) to about 100 000 dalton (the first fraction). Most of the humic molecules of bank filtrated lake water were found in the third and fourth fractions. After the oxidation and filtration processes the first fraction of humic molecules disappeared completely and the fourth and fifth fractions dominated. The total sum of the humic fractions was most effectively reduced after the O3/H2O2 treatment, by about 70%.