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The persulfate-based advanced oxidation process is a promising method for degrading organic pollutants. Herein, TiO₂ and ZnO photocatalysts were combined with the peroxydisulfate ion (PDS) to enhance the efficiency. ZnO was significantly more efficient in PDS conversion and SO₄^•− generation than TiO₂. For ZnO, the PDS increased the transformation rate of the trimethoprim antibiotic from 1.58 × 10⁻⁷ M s⁻¹ to 6.83 × 10⁻⁷ M s⁻¹. However, in the case of TiO₂, the moderated positive effect was manifested mainly in O₂-free suspensions. The impact of dissolved O₂ and trimethoprim on PDS transformation was also studied. The results reflected that the interaction of O₂, PDS, and TRIM with the surface of the photocatalyst and their competition for photogenerated charges must be considered. The effect of radical scavengers confirmed that in addition to SO₄^•−, ^•OH plays an essential role even in O₂-free suspensions, and the contribution of SO₄^•− to the transformation is much more significant for ZnO than for TiO₂. The negative impact of biologically treated domestic wastewater as a matrix was manifested, most probably because of the radical scavenging capacity of Cl⁻ and HCO₃⁻. Nevertheless, in the case of ZnO, the positive effect of PDS successfully overcompensates that, due to the efficient SO₄^•− generation. Reusability tests were performed in Milli-Q water and biologically treated domestic wastewater, and only a slight decrease in the reactivity of ZnO photocatalysts was observed.