Two kinds of activated carbon (AC) particles, which possess large adsorption capacity for gaseous ammonia and trichloroethylene (TCE), were selected as the carrier of TiO2 ultrafine powders. Commercially available TiO2 sol solutions were used as the source of TiO2 powders. For the sake of comparison, porous glass (PG) particles and alumina (AL) particles were also selected as the carrier. The photocatalytic activity of TiO2/AC, TiO2/PG, and TiO2/AL was examined using a circulation reaction method. The photocatalytic degradation of gaseous ammonia and TCE was found apparently to obey the first-order reaction with respect to ammonia or TCE. The first-order reaction rate constant over TiO2/AC was shown apparently to be higher than those over TiO2/PG and TiO2/AL. The observed relationship between the apparent first-order rate constant and the TiO2 catalyst loading could be interpreted satisfactorily in terms of a reaction model based on the premise that the degradation rate constant is proportional to the illumination intensity, whose decay obeys the Lambert-Beer law; this is an extension of the reaction model in a three-phase fluidized-bed reactor under illumination proposed in our preceding paper.