A new chronoamperometric method is used to estimate changes in the membrane composition of mobile-site, ionophore-based membranes. The characteristic features of the chronoamperometric curves (initial current, slope, break time) of valinomycin-based, potassium-selective membranes loaded with potassium tetrakis(4-chlorophenyl)borate are correlated with the mobile-site and free ionophore concentration in the membrane. limiting cases for strong and negligible ion pair formation are distinguished. Replicate measurements indicate a relative standard deviation in the calculated values less than 10%. The practical applicability of the method was tested with membranes incorporated into conventional ion-selective electrode bodies or cast onto microfabricated planar sensor structures.