The supercritically treated TiO2-activated carbon (Sc-TiO2-AC) composites were studied for their adsorption and subsequent photocatalytic activities for acetaldehyde. Sc-TiO2-AC composites showed much higher decomposing ability for acetaldehyde compared to a noncomposite comprising a simple mixture of activated carbon and TiO2. This indicates that Sc-TiO2-AC composites have a synergetic effect of adsorption and photocatalytic decomposition of acetaldehyde. Meanwhile, acetaldehyde was found to be decomposed to CO2 as final product by Sc-TiO2-AC composites through the conversion to methanol, formaldehyde and acetic acid as intermediates. It was also found that many OH groups exist on the Sc-TiO2-AC composites. These OH groups can be assumed to take part in the photocatalytic degradation of acetaldehyde.