We investigated a photocatalytic reaction on TiO2 nanotube (NT) surfaces using infrared absorption spectroscopy with multiple-reflection geometry (MIR-IRAS). We used an anodization technique to form a film of well-aligned TiO2 NTs on a Si prism used for MIR-IRAS measurements. The photocatalytic decomposition process of the endocrine disruptor, dioctyl phthalate (DOP), on the TiO2 NT surface was monitored in-situ and in real time. We demonstrated that the photocatalytic decomposition of organic materials is promoted with the presence of molecular oxygen. It was observed that the amount of surface-adsorbed water molecules changed during the reaction. We proposed a simple reaction model that can reproduce the time-dependent change of the surface coverage of water and DOP. By comparing the photodecomposition of organic materials on TiO2 NT films with that on TiO2 nanoparticle (NP) films, we showed that TiO2 NT films are superior in photocatalytic reactivity compared to NP films. We suggest that the NT structure provides wider and shorter paths for the transport of photo-generated radicals and byproducts, leading to a higher reactivity compared to TiO2 NPs. (c) 2019 The Electrochemical Society.