From electrochemical measurements at the interface of two immiscible electrolytes, the current at controlled potential is usually a linear function of the ion concentration in the aqueous phase. Surprisingly, a linear relationship between the current and the logarithm of the sample ion activity is found for corresponding measurements on ion-selective electrode membranes. Here, a theoretical explanation for the apparent contradiction between the behavior of the two kinds of system is given. Experimental results obtained with conventional ion-selective PVC membranes as well as with membranes based on PVC free membrane matrices are presented. © 2004 Elsevier B.V. All rights reserved.