UV light-activated highly efficient photocatalytic decomposition of ammonia to N-2 in water was successfully achieved by using only a simple mesoporous TiO2 thin film coated on a transparent substrate with external quantum efficiency of 35 (= 3.5 x 10(3)%) based on the total incident UV light. The internal quantum efficiency exceeded 250 (= 2.5 x 10(4)%) in a large scale 22.2 L submodule meaning that ammonia was once activated at TiO2 by UV light, the activated ammonia underwent more than 250 times oxidative decomposition by O-2 probably via a kind of chain reaction mechanism.