Separation of silver ions(Ag(I)) from aqueous solution by TiO₂(titanium dioxide) thin film photocatalysis was investigated. Disappearance of silver ions from solution was due to the reduction of silver ions as metal silver deposits on the surface of TiO₂ thin film during photocatalytic reactions. As the thickness of TiO₂ thin film increased, the amount of silver ions reduced to metal silver increased. Meanwhile, as silver deposits on TiO₂ thin films increased, the reduction rate of silver ions and the degradation rate of formic acid were decreased. It was found that the silver ions competed with oxygen for the electrons during photocatalytic reactions.