A decatungstate-containing bilayer composite was prepared by ion complexation betweendecatungstate anion (W10) and dioctadecyldimethylammonium (DODMA). The cast filmprepared from its chloroform solution had a well-organized bilayer structure and a phasetransition of the bilayer. The W10/DODMA composite powder dispersed in 2-propanolphotocatalyzed the oxidative dehydrogenation of 2-propanol into acetone in the presence ofoxygen and under UV irradiation. The W10/DODMA exhibited a unique temperature-dependent property that the photocatalytic activity is drastically enhanced above the phase-transition temperature of the bilayer. When the W10/DODMA composite was combined withPt nanoparticles, the photocatalytic dehydrogenation of 2-propanol into acetone and H2proceeded in the absence of oxygen because Pt promoted the reoxidation of protonatedH2W10O324- to the original W10O324- (H2 evolution step).