The photocatalytic degradations of an acid dye, C.I. Acid Blue 40, on titanium dioxide (TiO2) particles suspended in a gas-liquid dispersion, and of methylethyl ketone (MEK) on TiO2 particles suspended in the liquid were carried out using coaxial double-cylinder-type reactors. The former degradation was executed in a three-phase fluidized bed, whereas the latter one was done in a liquid-solid fluidized bed. A black-light-type fluorescent tube was installed in the internal cylinder of the reactor. The photocatalytic degradation of both acid dye and MEK here could be described as first-order with respect to the reactant species (i.e., acid dye or MEK). The apparent first-order rate constants in both systems, however, decreased with increasing initial concentration of reactant species. Such an observation was examined on the basis of the Langmuir-Hinshelwood-type catalytic reaction mechanism and the competition of light absorption due to the reactant species. In both degradation systems, the observed relationship between the apparent first-order rate constant and TiO2 catalyst loading was satisfactorily compared with the prediction on the basis of the premise that the decay of light illumination obeys Lambert-Beer law.