The electrochemical behavior of a cobalt-iron (CoFe) alloy electrode in 0.1 M Na2SO4 (sodium sulphate) aqueous solution of different pH was investigated. The presence of a different concentration of dissolved sulphide ions with different pH on the corrosion of electrodeposited cobalt-iron was investigated using the EQCM technique. The corrosion rates of the alloys in acidic solutions are pronouncedly high, compared to those measured in neutral or basic solutions. The presence of sulphide ions accelerates markedly the anodic reaction for all deposits in the basic medium. The dissolution rate was twice two times faster when S2− was added. By employing the EQCM, it was observed that the mass decrease or dissolution rate was significant with an increase in pH concentration.