Titania　sols　were　prepared　by　acid　hydrolysis　of　a　TiCl4　precursor　instead　of　titanium　alkoxides.　The　effect　of　acid　concentration　on　the　particle　size　and　stability　of　sol　was　investigated.　Stable　titania　sols　with　mean　particle　size　of　14　nm　could　be　obtained　when　the　H+/Ti　molar　ratio　was　0.5.　The　titania　sols　were　modified　with　Pt,　SiO2,　ZrO2,　WO3　and　MoO3　to　prepare　a　series　of　modified　catalysts,　which　were　used　for　the　photocatalytic　oxidation　of　formaldehyde　at　37°C.　They　showed　different　photocatalytic　activities　due　to　the　influence　of　the　additives.　Comparing　with　pure　TiO2,　the　addition　of　silica　or　zirconia　increased　the　photocatalytic　activity,　while　the　addition　of　Pt　and　MoO3　decreased　the　activity,　and　the　addition　of　WO3　had　little　effect　on　the　activity.　It　is　of　great　significance　that　the　conversion　of　formaldehyde　was　increased　up　to　94%　over　the　SiO2-TiO2　catalyst.　The　increased　activity　was　partly　due　to　higher　surface　area　and　porosity　or　smaller　crystallite　size.　A　comparison　of　our　catalyst　compositions　with　the　literature　in　this　field　suggested　that　the　difference　in　activity　due　to　the　addition　of　a　second　metal　oxide　maybe　caused　by　the　surface　chemistry　of　the　catalysts,　particularly　the　acidity.　©　2001　Elsevier　Science　B.V.　All　rights　reserved.