Simultaneous removal of antibiotic tetracycline hydrochloride (TC) and As(III) by poorly-crystalline Mn dioxide was investigated. TC and As(III) can be effectively oxidized and removed by MnO₂. High concentrations of TC and As(III) competed with each other for oxidation or adsorption sites on MnO₂ and thus affected their removal efficiency. The intermediates and products of TC after reaction with poorly-crystalline manganese dioxide were identified by LC–ESI–MS (liquid chromatography–electrospray ionization–mass spectrometry), and the decomposition pathways of TC by MnO₂ were proposed. This study is helpful for understanding the importance of environmental Mn dioxides in the decontamination of combined pollution by organic pollutants and metal(loid)s.