Two oxidation processes, O-3 and MnO2/O-3, for the degradation of precursors of trihalomethanes(THMs), humic and fulvic acids, in aqueous solution have been investigated in this paper. Total organic carbon (TOC) and chemical oxygen demand (COD) were the parameters followed to evaluate the performance of both processes of oxidation. Comparison of their effect on the enhancement of the biodegradability was studied. While ozonation was efficient to reduce 73% COD and 67% TOC at [O-3] = 47 mg/l, CODi = 205 mg/l and TOCi = 28 mg/l in 30 minutes, ozone in the presence of MnO2 was efficient to reduce 89% COD and 79% of TOC in 30 minutes at [O-3] = 47 mg/l, CODo = 180 mg/l and TOCi = 38 mg/l.