To develop a high-performance photocatalytic reactor for purification of indoor air, the photocatalytic decomposition of gaseous HCHO at a very low concentration is investigated both theoretically and experimentally. In this second paper, a photocatalytic reactor with a parallel array of nine light sources, designed on the basis of the experimental result in the first paper, is applied to the air containing HCHO at an indoor concentration level and the reactor performance is discussed. The experimental result indicates that this photocatalytic reactor can rapidly decompose HCHO toward zero concentration. The reason for this high reactor performance is explained by a mathematical model that takes into consideration a film-diffusional resistance in the neighborhood of the photocatalyst as follows: (1) the reaction field is irradiated with a high light intensity because the distance between the light source and photocatalyst surface is only 6 mm; (2) the rate of decomposition is increased by the UV light that permeates through a glass tube; (3) the film-diffusional resistance is remarkably reduced because of a high linear velocity (709 m min(-1)). (c) 2005 Elsevier B.V. All rights reserved.