Photocatalysis　is　an　emerging　and　promising　technology　for　indoor　air　purification.　This　photocatalytic　method　is　effective　in　the　case　of　a　higher　pollutant　concentration,　but　its　wide　application　in　indoor　air　purification　is　limited　due　to　the　low　level　of　indoor　air　contaminants.　In　order　to　improve　the　removal　of　pollutants　in　indoor　air,　we　evaluated　the　photocatalytic　performance　over　the　nanosized　TiO(2)　particles　immobilized　on　the　surface　of　activated　carbon　(AC)　filter　for　the　removal　of　formaldehyde　(HCHO).　It　is　shown　that　the　photocatalytic　reaction　rate　increased　because　the　AC　could　adsorb　the　pollutants　from　the　diluted　air　stream　to　generate　a　high　concentration　of　the　pollutants　on　the　catalyst　surfaces.　The　photocatalytic　reaction　took　place　from　the　diffusion　control　process　to　the　photocatalytic　reaction　control　process　with　the　rise　in　flow　velocity.　In　the　former　process,　the　photocatalytic　reaction　rate　increased,　whereas　in　the　later　process　photocatalytic　reaction　rate　changed　little　with　increasing　flow　velocity.　The　flow　velocity　was　lower　over　the　TiO(2)/AC　catalyst　than　over　the　TiO(2)/glass　catalyst　when　the　photocatalytic　reaction　was　switched　from　the　diffusion　control　process　to　the　photocatalytic　reaction　control　process.　It　is　also　observed　that　the　indoor　low-concentration　HCHO　could　be　photocatalytically　degraded　over　TiO(2)/AC,　with　the　HCHO　concentration　in　the　product　mixture　falling　into　the　standard　range　that　is　specified　by　the　indoor　air　quality　standard　of　China.　(C)　2009　Elsevier　Ltd.　All　rights　reserved.