AbstractThe stable phases in the systems FeAs, CoAs and NiAs obtainable by thermal synthesis and by precipitation with arsine have been examined by X‐ray diffraction and their electrochemical and thermodynamic behaviour compared.Precipitation of the compounds with Me/As ratio near to one is accompanied by hydrogen evolution whilst the metal rich derivatives obtainable only with nickel (Ni11As8, Ni5xAs2) are the result of an oxydo‐reduction reaction function of solution pH with as second member of the couple AsO 2−.Cathodic polarisation leads to metal rich compounds in the case of iron (Fe2As) and nickel (Ni11, As8and Ni5As2). In all cases excess metal is bound to the substrate in an amorphous non ferro