Abstract: The character of cerium oxide membrane ion-selective electrode (ISE) for individual and mixed rare earth system in the non-linear response region has been investigated.The potential of the linear response region of ISE for HCl-CeCl₃ system at pH=4.9-6.0 and rare earth conceniration (C)=10^-2-10^-5M can be described as: E=-30lgC_CeCl-47.1 The relations of △E-C and △E-pH for HCl-PrCl₃ system show that when pH<2 the interference is higher and the electrode can't be used for the measurement. The relations of △E-C for two rare earths mixed (Pr-Nd)Cl₃ C=0.25-0.7M, four rare earths (La-Ce-Pr-Nd) Cl₃ C=0.14-0.7M and fifteen rare earths (La-Lu) and Y C=0.2-1.0M systems in the presence of NH₄Cl show that when pH≥ 2 the responses similar to those what in the individual rare earth, while pH<2 the interference of the acidity is evident. A general, more accurate model for various systems is proposed through mathmatical simulation: E=B₁lg[C_i +B₂(C_i+C_j)^B₂]