Abstract: Dotarem is the most used gadolinium-based contrast agent and one of the main sources of anthropogenic gadolinium in water systems. A Dotarem aqueous solution at its excretion level concentration was photocatalyzed by different materials. A 42% conversion was reached by photolysis, followed by photocatalysis with TiO₂, iron oxides, and TiO₂/γ-Fe₂O₃ (39–29%). Transformation products were identified to correspond to hydroxylated-Dotarem derivatives, Gd-free compounds, and Fe-complexes. A conversion pathway was proposed to demonstrate that, under UVC irradiation, Dotarem released toxic Gd³⁺, and in the presence of Fe-containing materials, transmetallation reactions occurred between the Gd³⁺ and Fe³⁺ ions.