Moisture and aggressive ion ingress into bonded joints are primary causes of adhesive degradation. In this study, moisture diffusion behavior of aluminum-powder-filled epoxy adhesive was investigated through utilizing fluid immersion tests under complete immersion in salt solutions with varying NaCl concentrations. Aluminum powder is used in the adhesive for the purpose of improvement of its thermal properties, as demanded in a variety of industrial applications. Mass diffusivity for each specimen was determined by two methods, one using the diffusion data at early times (away from the saturation point) and the other using the data at large times (close to the saturation point). The results of the two methods were quite different, indicating that diffusivity is concentration dependent and a constant diffusivity assumption might lead to error in determining moisture diffusivity values in epoxy systems. Qualitatively, however, both methods indicated similar diffusion behavior. According to the results of both methods, the aluminum filler content did not affect the moisture diffusivity in the epoxy adhesive significantly but the effect of salt concentration was significant; the higher the salt content in the test solution, the higher the moisture diffusivity in the adhesive.