High and low frequency dielectric measurements are reported on adhesively bonded aluminum structures aged by exposure to moisture over an extended period of time. The changes observed are interpreted in terms of initially water diffusing into the adhesive layer and there is little evidence of changes occurring to the surface oxide layer. Differences in the form and rate of change of the dielectric signature reflect the effects of the pretreatment and matrix resin on the water diffusion process. A good correlation is observed between the time domain data and the increased dielectric permittivity resulting from ingress of moisture into the adhesive bend. The paper illustrates the use of high frequency dielectric measurements in non-destructively probing adhesive bonds and indicates the potential for in-situ applications.