Water samples collected in various sites of a corn. H(2)O treatment plant (on inlet, after oxidn. with O(3) after filtration, after disinfection) were studied for COD, oxidizability color, turbidity, pH, as well as for contents of ammonium N, phosphates, assimilable and dissolved biodegradable org. C (AOC and BDOC, resp.). The AOC content was detd. by the van der Kooij method under use of Pseudomonas fluorescens bacteria. The BDOC content was detd. by the Joret method under use of microorganisms taken of from the biolog. bed. The oxidn. with O(3) resulted in decrease in AOC and BDOC contents after the treatment and in an improvement of the H(2)O quality.