A synergistic effect of photocatalysis and ozonation on the decomposition of formic acid dissolving in an aqueous solution has been studied. In the photocatalysis over a thin film of titanium oxide immobilized on the inner surface of a glass tube, a 6 W blacklight blue fluorescent lamp (wavelength: 300-400 nm) was used as a light source. The initial decomposition rates followed a Langmuir-Hinshelwood type and the hydrogen peroxide generated during the photocatalytic reaction played an important role in the decomposition of formic acid. In the ozonation, a 6 W low-pressure mercury lamp (wavelength: 185 nm) was used to produce ozone by irradiation of the air with UV When this air was circulated in a closed system with a water-holding glass container, the ozone concentrations in the air and water reached 0.350 g m(-3) air and 0.037 g m(-3) liquid, respectively, at maximum. The decomposition rate of formic acid by ozone was higher for a lower liquid temperature and a higher pH value. For comparison, the Langmuir-Hinshelwood type was also used to analyze both the experimental values obtained in the ozonation alone and in the combination of photocatalysis and ozonation. A relationship between the reaction rate and reactant concentration was calculated using the kinetic parameters determined from the experimental values in each reaction system. As a result, the decomposition rate of formic acid by the combination of photocatalysis and ozonation was found to be 31% higher at maximum than the sum of the decomposition rates when formic acid was individually decomposed by the two methods, which indicates the presence of a synergistic effect of the photocatalysis and ozonation. This effect may be explained by the promoted production of hydroxyl radicals by ozone over titanium oxide.