The anions PCO– and AsCO– are shown to be electroactive and are studied in aqueous and non‐aqueous solutions. Cyclic voltammetry is used to extract fundamental physicochemical parameters such as oxidation peak potentials, and transfer and diffusion coefficients of the anions to better understand the nature of the oxidation process. Variation of the potential scan rate reveals that electro‐oxidation of PCO– with the release of CO is controlled by diffusion and is a one‐electron irreversible process yielding phosphorus‐containing deposits. In contrast, the oxidation of AsCO– is a near electrochemically reversible process, forming pure arsenic deposits, with a chemically irreversible follow‐up reaction. For both anions, the electrode surface is substantially “blocked” by the reaction products. The formed deposits were characterized by scanning electron microscopy and energy dispersive X‐ray spectroscopy.