The oxidation of drugs using metalloporphyrins has been the subject of several studies in recent years. Diclofenac, one of the most frequently used anti-inflammatory drugs, is metabolized in humans by cytochrome P450 (CYP) enzymes to hydroxy-derivatives and to some metabolites resulting from oxidative decarboxylation. In this paper, the in vitro formation of several new diclofenac derivatives, initially resulting from oxidative decarboxylation, similar to what happens in vivo, is revealed. Chloro [5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrinato]manganese(III), [Mn(TDCPP)Cl], and chloro [5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]manganese(III), [Mn(TPFPP)Cl], are tested in the presence of hydrogen peroxide at room temperature. The new products obtained are fully characterized, three of them being characterized in the solid state using X-ray diffraction studies.