This work aims to resolve some controversies about astatine(III) hydroxide species present in oxidant aqueous solution. AtO+ is the dominant species existing under oxidizing and acidic pH conditions. This is consistent with highperformance ion-exchange chromatography data showing the existence of one species holding one positive charge. A change in speciation occurs as the pH changes from 1 to 4, while remaining under oxidizing conditions. Dynamic experiments with ion-exchange resins evidence the existence of a neutral species witnessed by its elution in the void volume. Batch-experiments using a competition method show the exchange of one proton indicating the formation of the AtO(OH) species. The hydrolysis thermodynamic constant, extrapolated to zero ionic strength, was determined to be 10−1.9. This value is supported by two-component relativistic quantum calculations and therefore allows disclosing unambiguously the structure of the formed species.