Purpose: This study examined the ultrastructure and microtensile bond strengths (TBS) of self-etching (with different acidity) and conventional adhesive systems bonded to unground enamel. Materials and Methods: Resin composite (Filtek Z250) buildups were bonded to unground enamel surfaces of third molars after adhesive application with the following materials: Clearfil SE Bond (CSE); Optibond Solo Plus Self-Etch (OP); Tyrian Self Priming Etching (TY), and the controls Scotchbond Multi-Purpose Plus (SBMP) and Single Bond (SB). Six teeth were assigned to each material. After storage in water for 24 h at 37°C, the bonded specimens were sectioned into beams of approximately 0.8 mm2 and subsequently subjected to µTBS testing at a crosshead speed of 0.5 mm/min. The average values were subjected to one-way ANOVA (α = 0.05). The effect of surface conditioning of each material was observed under scanning electron microscopy (SEM). Results: The highest resin-enamel bond strength was observed for SBMP (22.7 ± 5.2) and SB (26.7 ± 5.2 MPa). The lowest mean bond strengths were 10.9 ± 3.2 and 7.8 ± 1.5 MPa for TY and OP, respectively. CSE showed an intermediate performance (18.7 ± 4.6 MPa). An overall increase in porosity was evident along the entire enamel surface treated with the self-etching primers; however, no selective demineralization similar to that with 35% phosphoric acid was observed. Conclusion: The highest bond strength means and the more retentive etching pattern were observed for the two-step etch-and-rinse adhesives. Among the self-etching systems studied, Clearfil SE Bond should be preferred. Palabras clave: adhesive system, bonding, enamel morphology