To develop a high-performance photocatalytic reactor for purification of indoor air, photocatalytic decomposition of HCHO at a very low concentration is investigated both experimentally and theoretically in two papers. This first paper reports the result of fundamental experiment that is necessary to design the photocatalytic reactor in the second paper. A combination of the absorption of trace HCHO into water with the quantitative analysis of HCHO by the 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole (AHMT) method is found to be useful to accurately measure the HCHO concentration at a ppb level (less than 1.23 mg m(-3)). The experimental results obtained using three types of annular-flow photocatalytic reactors with different inside diameters (22-120 mm) and light sources (6 and 20 W blacklight blue fluorescent lamps) indicate that the photocatalytic reactor with a 6 W lamp and a glass tube of 28 mm in inside diameter (the distance between the light source and photocatalyst surface; 6.5 mm) gives a maximum rate of decomposition; HCHO at a ppb concentration level is rapidly decomposed to zero concentration when the photocatalyst surface kept at a low temperature (45 degrees C) is irradiated with UV light of a high light intensity. (c) 2005 Elsevier B.V. All rights reserved.