Catalytic ozonation of a filtered surface water with ferric hydroxide (FeOOH) was compared with ozonation alone for trihalomathane formation potential (THMFP). Factors studied included bromide, pH, alkalinity, ratio of O3/TOC, reaction time, and dosage of FeOOH. THMFP of the water after catalytic ozonation was 30.5% lower than that after ozonation. High bromide concentration resulted in the predominance of brominated THMFP, at which condition the brominated THMFP after catalytic ozonation was 45%-65.5% of that after ozonation. The catalytic ozonation showed a significant advantage over ozonation in controlling THMFP of the water at the conditions applied in this experiment, i.e., at pH of 6.33-9.43, O3/TOC of 0.65-2.05, and reaction time of 2-20 min. High concentration of alkalinity decreased THMFP of the water after ozonation and catalytic ozonation. It also weakened the advantage of catalytic ozonation. There existed an optimal dosage of FeOOH in respect of controlling THMFP. The lower THMFP of the water after catalytic ozonation is caused by its higher TOC removal than ozonation, and the possible enhanced oxidation of some of the THM precursors by hydroxyl radicals generated in this process.