The present research attempts to non-destructively characterize mechanical properties, which are representative of the interfacial adhesion and bond line cohesion of adhesively bonded assemblies, using an ultrasonic method. Eight bonded samples made of two aluminium substrates and of an epoxy-based adhesive layer were manufactured: four in which the adhesive is fully cured (100%) and four in which crosslinking is partial (80%). For each level of curing, four different surface treatments were applied to the aluminium substrates before assembling, in order to vary the quality of adhesion. Ultrasonic plane wave transmission coefficients (UPWTC) were either measured in a water tank, or simulated using the well-known stiffness matrix method that micmics the experiments, to produce input data for the inverse problem. This latest consists in the evaluation of the elastic properties of either the adhesive bond or the interphases between both substrates and the adhesive layer. If the interphases are of nominal quality, the values of the inferred properties of the adhesive bond match those previously measured on individual epoxy samples, whether the epoxy is fully or partially cured. However, when interphases are not of nominal quality level, the optimized bond moduli reveal an apparent anisotropy, although the epoxy layer is known to be isotropic. This apparent anisotropy is explained by an analytical rule of mixture, thus giving confidence in the proposed ultrasonic technique, which is then suggested as a potential way to detect weaknesses of interphases. Finally, the optimization of the interphases elastic properties is carried out. The measured normal and shear stiffnesses are shown to decrease as the interphases get degraded. All ultrasonically-measured parameters (apparent anisotropy of the bond and interfacial stiffnesses) vary monotonically with the bonds strength, which was measured via mechanical tests. The proposed UPWTC method was shown to have a strong potential to distinguish between adhesive and cohesive weaknesses of bonded joints, and to estimate corresponding mechanical properties.