The purpose of this study was in vitro evaluation and comparison of the adhesion of self-etch (SE) adhesive systems applied on normal and sclerotic dentin. For this study, Class 5 cavities were prepared on sound teeth as well as on teeth with sclerotic dentin. They were then restored by means of the SE 2-step OptiBond XTR (Kerr) and SE 1-step Bond Force (Tokuyama Dental) adhesive systems, as well as the Estelite Sigma Quick (Tokuyama Dental) composite resin. For teeth with sclerotic dentin, the hypermineralized superficial layer was removed by means of round bur on low speed, than the adhesive systems and composite resin were applied. These teeth were prepared for microscopic study according to the protocol specific to each microscope. SEM (scanning electron microscopy) examination reveals that on normal and sclerotic dentin, OptiBond XTR and Bond Force form hybrid layers with about the same thickness, greater in normal dentin, but only OptiBond XTR pervades into the dentinal tubules, both in normal and sclerotic dentin. However, TEM (transmission electron microscopy) examination of Bond Force reveals that it penetrates into the dentinal tubules as well, but only in the case of normal dentin. The thickness of the hybrid layers resulting from the application of the SE adhesive systems to sound dentin is different from the thickness of the hybrid layers obtained when the same adhesive systems have been applied to sclerotic dentin.