An in-situ sensor, based on Electrochemical Impedance Spectroscopy (EIS), has been used to monitor the health of adhesive bonds constructed from various combinations of aluminum, graphite/epoxy, glass/epoxy, glass/polyester, and glass/vinylester composites and exposed to high humidity and temperature conditions. Modeling of the EIS data as an electric circuit demonstrated that several circuit parameters of the impedance spectra were sensitive to bond performance, as determined by wedge tests and lap shear tests. Moisture absorption by the adhesive and composite was calculated from the circuit capacitance, which was also a function of bonded area and bondline thickness (bondline + composite thickness for glass composites). Material differences, including saturation level of moisture, rate of absorption, and bondline stability, were readily seen among the various materials sets. The sensor electrodes are attached to opposite sides of a bond after fabrication, i.e. they are not embedded. Thus, they are suitable for monitoring existing bonded structures. They have the potential to identify bondlines that are in the early stages of degradation, prior to significant loss of bond strength. As an input to a condition-based maintenance system, they would identify weakening bondlines and allow preventative action to be scheduled and performed.