The photocatalytic decomposition of acetaldehyde in air at initial concentrations ranging from 3 to 200 mg m(-3) has been studied in a semitransparent closed box with an inlet volume of 0.056 m(3). The photocatalytic reactors consisted of a glass tube, 250 mm long with inside diameters of 28, 35, or 45 mm, whose inner surface was coated with a thin film of titanium dioxide, and a 6-W blacklight fluorescent lamp located at the axis of the glass tube. The decomposition of acetaldehyde was almost complete within 1-3h and its main product was carbon dioxide. A kinetic study showed that the photocatalytic reaction obeys a Langmuir adsorption isotherm. Although the light intensity was certainly decreased with the distance from the light source, the degree of this decrease was much smaller than the degree of the decrease in the kinetic constants, which suggests that the light intensity is not simply proportional to the degree of the photo-excitation of TiO2 and the rate of the resulting photocatalytic decomposition of acetaldlyde,