The coccidiostat monensin can be determined by voltammetry at the interface between two immiscible electrolyte solutions (ITIES) in the concentration range between 0.05 mM to 3 mM. The peaks obtained with cyclic voltammetry at the water/nitrobenzene interface correspond to the transfer of sodium ions from the aqueous to the nonaqueous phase facilitated by complexation with monensin functioning as an ionophore and are proportional to its concentrations. The method was applied to the extracts fromStreptomycescultures.