Hydrothermally prepared titanate nanotubes are known to be thermodynamically unstable and to be easily transformed to the titania phase by heat or acid treatment, which is accompanied by the destruction of the nanotube structures. Surface modification of titanate nanotubes is expected to retard the phase-transition reactions from titanate nanotubes to titania nanoparticles. In this study, we investigated the effects of surface modifications, such as fluorination or phosphation, on the phase-transition reactions of hydrothermally prepared titanate nanotubes under acidic conditions. After acid treatment, surface-modified titanate nanotubes maintained their nanotube structures, and spherical nanoparticles were partially formed in the titanate nanotubes. Unmodified titanate nanotubes, however, changed to elongated nanostructures and spherical nanoparticles. The surface modifications also strongly affected the crystalline phases and increased the BET surface area. The photocatalytic activities for methylene blue degradation and gaseous CH3CHO oxidation were strongly enhanced by the surface modifications. These effects were attributed to the anatase crystalline phase and the high surface area. Therefore, these surface modifications are useful methods to improve the photocatalytic activities of titanate nanotubes. (C) 2012 Elsevier B.V. All rights reserved.