OBJECTIVE To evaluate in vitro the qualitative characteristics of the surfaces of some enamel-dentin bonding systems. METHODS Seven different bonding systems were observed first by stereomicroscopy then by scanning electron microscopy, as follows: 49 extracted teeth were abraded to expose a uniform dentin surface. After application of the bonding system, composite resin was placed on the surface and shaped as an inverted truncated cone. The tensile bond test was performed and the specimens observed under SEM; the fracture location was determined and the formation of the hybrid layer evaluated. RESULTS SEM observation indicated that adhesive, cohesive and mixed fractures had occurred. The abraded dentin surfaces were also observed under SEM before and after etching. The most recent enamel-dentin bonding systems use a conditioning technique that demineralizes the upper dentin surface at and around the tubule entrance. This is known as the total etching technique. A layer is formed in which the demineralized dentin is impregnated with fluid resin: the so-called hybrid layer. CONCLUSIONS The length of the plugs of adhesive was not found to be determinant. The formation of the hybrid layer appeared to be more important, as it indicated a three-dimensional interaction between the conditioned collagen fibres and the primer that diffuses the adhesive.