A method for the removal of sulfate ions from an electrolyte of an electrochemical reduction-oxidation system wherein said electrolyte comprises a halogen and in which the sulfate ions an a contaminant or interferant, which method comprises the steps of: (i) increasing the halide concentration in the electrolyte by electrochemical reduction of the halogen, (ii) crystallizing a sulfate salt out of the electrolyte, and (iii) separation of the electrolyte from the crystallized sulfate salt. An electrochemical process for energy storage and/or power delivery comprising the step of removal of sulfate ions from the electrolyte according to the above cited method is also disclosed.