New polymeric sensors based on bidentate neutral organophosphorus compounds of varied structure with addition of chlorinated cobalt dicarbollide, which are highly sensitive to Fe³⁺, Y³⁺, La³⁺, Pr ³⁺, Nd³⁺, and Eu³⁺ cations in a wide range of concentrations at pH 2, were suggested. The selectivity, detection limits, and pH-dependence of the potential of the sensors developed were studied. The influence of chlorinated cobalt dicarbollide on the electrochemical characteristics of the sensors was demonstrated.