Effects of the use of inert supports for TiO2 loading on photocatalyzed decomposition of propionaldehyde in the gas phase was investigated for mordenite support with various amounts of TiO2 loading and for several kinds of supports such as other zeolites, alumina, silica, and activated carbon. The adsorption constant and the amount of adsorption of propionaldehyde were evaluated for TiO2-loaded supports by obtaining Langmuir adsorption isotherms. By correlating these parameters to the photodecomposition rates of propionaldehyde, the involvement of the support in the photodecomposition reaction is clarified. The photocatalytic activity of TiO2 on mordenite having various amounts of TiO2 loading increases with increase in the amount of loaded TiO2 up to an optimum value (ca. 50 wt %), beyond which a decreasing tendency of the activity appeared. In the region of ascending activity, plenty of adsorbed substrate is available and the activity is controlled by the content of TiO2, while in the region of descending activity, the decrease in the amount of adsorbed substrate due to a decrease in the occupancy of the support by the TiO2 loading is responsible for the activity decrease. The photocatalytic activities are greatly influenced by the kind of inert supports used and show a volcano type dependence on the adsorption constant of the TiO2-loaded supports. In cases where the adsorption constant is low, the decomposition rate is determined by the amount of adsorbed substrate, while if the adsorption constant is very high, plenty of adsorbed substrate is available on the support, but it is not mobile to the loaded TiO2. We conclude that the use of an inert support having a medium adsorption constant is necessary to obtain the highest activity, where a high amount of adsorbed substrate that can be supplied to TiO2 particles is available.