Formaldehyde is a common indoor air pollutant and its removal by photocatalysis is investigated. Similar to other pollutants, however, formaldehyde is not the only pollutant which exists in the atmosphere but no study has yet reported the effects of the presence of other pollutants on the photodegradation of formaldehyde. In this study, common indoor air pollutants, namely nitrogen monoxide (NO), sulfur dioxide (SO2) and volatile organic compounds (VOCs) were selected to concurrently photodegradate with formaldehyde to identify their effects on formaldehyde. Formic acid was identified as the intermediate from the photodegradation of formaldehyde. The conversion of formaldehyde and the yield of formic acid decreased with increasing humidity levels. The presence of NO promoted the conversion of formaldehyde. The yield of formic acid was also higher under the presence of NO. Sulfate ion formed from the presence Of SO2 inhibited the conversion of formaldehyde and the yield of formic acid. Similar inhibition effect is observed from the concurrent photodegradation of VOCs and formaldehyde. Intermediates generated from the photodegradation of VOCs inhibited the conversion of formaldehyde. (C) 2004 Elsevier B.V. All rights reserved.