The light intensity, one of the most important factors for photocatalysis, directly influences the photocatalytic reaction rate and the utilization ratio of energy. Its rational selection has vital significance for saving energy. In the present paper, light energy losses in the heterogeneous photocatalysis process were analyzed. A method was presented to determine appropriate light intensities for the photocatalytic degradation of indoor gaseous volatile organic compounds (VOCs) based on the degradation mechanism. For simplifying the method, an excess coefficient of light intensity was defined based on the assumption of complete utilization of excited electron-hole pairs. The excess coefficients and the appropriate light intensities for degrading formaldehyde were determined according to the data from a titania-coated glass plate reactor and a titania-coated reticular foam nickel reactor. The results showed that the derived light intensities according to our method were appropriate. Good degradation performance and high utilization ratio of light energy can be attained simultaneously when the excess coefficient was 1.3-1.5 for formaldehyde with the concentration from 20.45 mu mol/m(3) (0.5 ppm) to 81.8 mu mol/m 3 (2 ppm), which is the concentration range of formaldehyde in "sick buildings". (c) 2006 Elsevier Ltd. All rights reserved.