Oxytetracycline (OTC) is an antibiotic used in aquaculture able to complex with the major cations Ca2+ and Mg2+, existing in brackish water. The effects of these cations on the solar photodegradation of OTC have been investigated. Calcium promotes a faster OTC photo-degradation in aqueous solution, while the same does not occur with Mg2+. For some authors the accelerating effect of Ca2+ is attributed to the occurrence of self-sensitized photodegradation of the complexes with Ca2+, which is not possible in the complexes with Mg2+, due to different binding patterns of these cations. In this work, the controversy concerning the binding sites of these cations in OTC is discussed. Based on spectroscopic evidence, similar binding patterns of OTC with Ca2+ and Mg2+ and a lower quantum yield of the direct photolysis of the complexes with Mg2+ are proposed. In addition, it was demonstrated that Mg2+ inhibits the formation of some OTC photoproducts observed in the presence of Ca2+, while at least two new OTC photoproducts are formed in the presence of Mg2+.