Acetaldehyde in air is known to be one of the primary indoor air pollutants and the most abundant carbonyl compounds in the atmosphere. Therefore, the photocatalytic oxidation (PCO) of gaseous acetaldehyde has been frequently studied as a test reaction in the studies of photocatalytic air purification. Polyoxometalates (POMs) as a homogeneous photocatalyst successfully demonstrated their activity to degrade aquatic pollutants and so did the silica-immobilized POM (POM/SiO2) as a heterogenized photocatalyst in water. In this study, the performance of POM/SiO2 on the PCO of acetaldehyde under the ambient air condition was investigated. The catalysts were prepared using tetraethoxysilane as a silica precursor and commercial POM powder without further purification by the sol-gel hydrothermal method. After the acetaldehyde adsorption equilibrium was attained on the catalyst surface in the dark, the irradiation of UV light was immediately followed and then the concurrent generation of carbon dioxide and removal of acetaldehyde was monitored by GC analysis as a function of reaction time. Pure POM powder, bare silica, and POM/SiO2 were compared for the adsorption and the photodegradation of acetaldehyde. The silica-immobilized POMs (Na3O40PW12, H3Mo12O40P, and H4O40SiW12) exhibited marked activities for the decomposition of pre-adsorbed acetaldehyde under UV irradiation whereas pure POM powders were little active. The effects of various experimental parameters were investigated in detail.