A strengthening system with adhesively bonded CFRP reinforcement was developed in the Infravation project FASSTbridge to extend the remaining fatigue lifetime of existing steel bridge constructions. The premature debonding of CFRP and steel is seen as a major risk to lose the contribution of the retrofitting system. Consequently, an ultrathin debonding sensor to be integrated in the adhesive layer between steel and CFRP was developed as part of the strengthening system. It is suited for continuous monitoring of the integrity of the adhesive joint. The paper explains the underlying physical principles of the sensor and gives insight into the performed tests for the calibration and verification of its functionality. The sensor conception takes the change of a capacitor's electrical impedance as a basis, which occurs when a change in distance between the adjacent conductive faces takes place. It is well assumed that debonding yields separation, which leads to an increase of the monitored impedance. The relationship is explored through single lap sheer experiments and vertical tension bond tests. The sensitivity and the accuracy of the impedance with respect to deformations are revealed by comparing several groups of prototypes with different dielectric layers and viscous materials. The influences of different temperature effects and fatigue loadings are also presented.