Catechol oxidase catalyses the oxidation of a broad range of catechols to the corresponding o-quinones through the four-electron reduction of molecular oxygen to water, without acting on tyrosine. The resulting highly reactive quinones autopolymerize to form brown polyphenolic catechol melanins, a process thought to protect the damaged plant against pathogens or insects. This work describes the catalytic activity of some radiopharmaceuticals like Tc-99m pertechnetate, Tc-99m mercaptoacetyltriglycine (MAG3), Tc-99m nanocolloid, Tc-99m methylenebis(phosphonic acid) (MDP) and Tc-99m methoxyisobutylisonitrile (MIBI) for oxidation of catechol. The catalytic oxidation of 3,5-di-tert-butylcatechol by selected radiopharmaceuticals saturated with molecular oxygen was studied in a homogeneous system. All the radiopharmaceuticals exhibited catecholase-like activity.