High levels of ozone (typically 850ppm) are readily decomposed by semiconductor photocatalysis, using a thin film of the semiconductor titanium dioxide (Degussa P25 TiO 2 ) cast on a glass tube, and UVA light, i.e. light of energy greater than that of the bandgap of the semiconductor (ultra-bandgap light); in the absence of this light the thermal decomposition of ozone is relatively slow. The semiconductor films show no evidence of chemical or photochemical wear with repeated use. At high levels of ozone, i.e. 100ppm=<[O 3 ]=<1400ppm, the initial rate of ozone decomposition by semiconductor photocatalysis is independent of [O 3 ], whereas, at lower ozone concentrations, i.e. 5ppm=<[O 3 ]=<100ppm, the initial rate of ozone photodestruction decreases in a smooth, but non-linear, manner with decreasing [O 3 ]. The kinetics of ozone photodecomposition fit a Langmuir-Hinshelwood type kinetic equation and the possible mechanistic implications of these results are briefly discussed.