Conventional methods to investigate ionophore concentration require complex laboratory equipment. One approach to reduce the size of devices for the biosensor application is the implementation of the sensor circuits on CMOS microchips. Thus a new approach for the detection of changes in the ionophore concentration in tethered Bilayer Lipid Membranes (tBLMs) implemented in CMOS has been developed. In case of ion transport across the membrane by ionophores the ion concentration in the spacer region of the membrane is increased. This causes a permittivity increase and therefore a change of the capacitance of the spacer region which can be measured. The advantages of the capacitive biosensor principle are label-free measurements with short measurement times in the millisecond range. The test of the sensor chip with a circuit that mimics the membrane shows very good agreement between simulation and measurement. In addition, a Valinomycin concentration of 100 nM has been detected. An estimation of the detection limit gives a value of about 10 pM.