The aim of this study is to propose a successful method for the treatment of water contaminated by pharmaceutical pollutants through homogeneous photocatalysis in the presence of decatungstate ions (W 10 O 32 4-). Sulfamethazine (SMZ), a sulfonamide antibiotic, was used as a model molecule. The results showed that SMZ could be effectively degraded with this process under simulated solar irradiation. SMZ degradation kinetics were studied with different dioxygen and SMZ concentrations, pH values, and photocatalyst masses. Optimal conditions were determined to be pH 7, [Na 4 W 10 O 32 ] = 0.33 g/L, and [SMZ] = 13.9 mg/L under the aerated condition, resulting in 85% SMZ degradation in 240 min, using a 36W-UVA/UVB light source. Hydroxyl radicals were identified as the major contributors to SMZ elimination. Four photoproducts identified with high-performance liquid chromatography coupled with mass spectrometry were formed by the cleavage of the sulfonamide bond and the hydroxylation of both the aromatic ring and pyrimidine moiety. SMZ was completely mineralized after 90 h of irradiation in the presence of decatungstate anions. These results provided a mechanism for the photocatalytic degradation of SMZ in an aqueous solution. To sustain this mechanism, theoretical studies were carried out using density functional theory calculations. This involved Fukui functional analyses, including ring hydroxylation, C-S bond cleavage, and molecular rearrangement processes.