At − (astatide) is commonly expected to be the heaviest halide in the halogen group. However, there is no proof for the existence of this −1 charged species. Furthermore, investigations with astatine are restricted by its specific radioactive properties, which entail working at ultratrace concentrations (typically less than 10 −10 M). In this work, an especially built electromigration device is applied to obtain information about the charge/size ratio characterizing an ion in aqueous solution. An anionic At species is observed in reducing conditions. Moreover, we propose the first absolute mobility value for the astatine species in acidic reducing condition: (−8.26 ± 0.59) × 10 −4 cm 2 ·V −1 ·s −1. This value appears close to that of I − ((−8.30 ± 0.33) × 10 −4 cm 2 ·V −1 ·s −1), which is obtained by the same method. The similar absolute mobilities obtained for both ions are coherent with theoretical calculations indicating similar diffusion behaviors for At − and I −. This good agreement confirms the existence of the At − species.