Composite nanoparticles (NPs) containing a TiO₂ outer shell and an inner core of hollow AuAg NPs (H-AuAg-NPs) were prepared using hollow AuAg NPs with block copolymer shells as templates, and their photocatalytic activities were investigated by the decomposition of methylene blue dye under UV irradiation. It was found that the H-AuAg-NP–TiO₂ composite NPs showed higher photocatalytic activity than AuAg alloy–TiO₂ composite NPs, Ag/TiO₂ NPs and TiO₂ NPs. In addition, the photocatalytic activity first increases with increasing the TiO₂ content from 2.5% to 5.0% and then decreases with a further increase in TiO₂ content from 10.0% to 15.0%.