The objectives of this study were to determine the optimal ozone conditions for the biological stabilization of drinking water in the subsequent BAC filtration and to provide an understanding of the relationship between NOM characteristics and process performance. A chemically pretreated and sand-filtered lake water was pumped into two parallel pilot-scale reactors, operated with ozone gas flow countercurrent to the continuous water flow. A 5x4x3 test matrix consisted of five realistic ozone doses (0.2-1.0 mgO3/mgTOC), four pH values (6.3-8.0) and three ozone contact times (4-10 minutes). Fractionation of NOM by high-performance size-exclusion chromatography indicated that ozonation removed preferentially the higher apparent molecular size (AMS) fractions depending strongly on the applied ozone dose. However, the ozone contact time and pH had no significant effect on the molecular size distribution. Similarly, only the transferred ozone dose had a significant effect on the reduction of UV absorbance (254 nm), which increased from 37 to 70 percent as the applied ozone dose increased from 0,2 to 1,0 mgO3/mgTOC. Ozonation at all the doses significantly increased the assimilable organic carbon (AOC) concentrations. In this study, a short contact time of 4 minutes proved to be most efficient for the biodegradable NOM formation. Longer contact times resulted in lower concentration of AOC, probably due to mineralization of biodegradable NOM. Efforts to demonstrate correlation between AOC and AMS fractions were unsuccessful.