Titania　sols　were　prepared　by　acid　hydrolysis　of　a　TiCl4　precursor　and　mixed　oxides　SiO2-TiO2,　ZrO2-TiO2,　WO3-TiO3　and　MoO3-TiO2　as　well　as　Pt/TiO2　catalysts　for　the　photocatalytic　oxidation　of　formaldehyde　were　obtained　by　using　the　method　of　the　modification　of　sol　particles.　The　influence　of　the　additives　on　the　photocatalytic　activity　was　investigated.　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.　Among　these　catalysts,　the　activity　of　the　SiO2-TiO2　catalyst　was　the　best　while　that　of　the　MoO3-TiO2　catalyst　was　the　worst.　It　is　of　great　significance　that　the　conversion　of　formaldehyde　was　increased　up　to　94%　over　the　SiO2-TiO2　catalyst.　The　increased　activity　of　the　SiO2-TiO2　catalyst　was　due　to　the　higher　surface　area　and　porosity,　the　smaller　crystallite　size　and　the　stronger　adsorption　of　UV　light.　A　comparison　of　our　catalyst　compositions　with　those　in　the　literature　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.