Pyrantel pamoate is a combination of pyrantel and pamoate used as a deworming agent for domesticated animals. This study investigated the sorption and transformation of pyrantel pamoate by birnessite (δ-MnO₂) under different conditions. Pamoate was first oxidized into keto naphthalene derivatives by manganese oxide. The pamoate degradation rate obeyed the second-order kinetic law, first order with respect to MnO₂ and first order with respect to pamoate. Pyrantel was not transformed by birnessite but showed adsorption on birnessite only after 20 h of reaction time when pamoate was transformed into products that could not participate in the solubilization of pamoate. The presence of natural organic matter showed inhibition of pamoate transformation and reduced pyrantel adsorption probably by competition for reactive sites and by enhancing water solubility of pyrantel. In soils and sediments, manganese oxide can participate in the oxidation of pamoate and sorption of pyrantel.