A new potentiometric method is proposed to measure apparent complex formation constants of ion carriers directly within ion-selective electrode membranes. This is accomplished with ion-selective electrode membranes containing a neutral H+ ionophore in addition to the ion carrier to be characterized. The difference in potential in the activity range of cation interference of H+-selective membranes with and without the ion carrier is used to calculate the effective stability constant. Experimental results with the potassium carrier BME-44, a lipophilic biscrown ether, are presented that correlate well with data obtained previously with a recently established spectrophotometric method.