To measure the buffering capacity of self-etching adhesive systems and to evaluate the effect of smear layer thickness on the mean gap width (GW) and microtensile bond strength (µBS) to dentin of these self-etching adhesives. Clearfil SE Bond (SE), Optibond Solo Plus Self-Etch Primer + Optibond Solo Plus (SO), Tyrian Self Priming Etchant + One Step Plus (TY) and as controls, Single Bond (SB) and ScotchBond Multi-Purpose Plus (SC) were used with Filtek Z250 Universal Restorative. The self-etching primers and 35% phosphoric acid (0.5 ml) were titrated by adding successive 0.05 ml drops of NaOH (1N). Thirty molars (n = 5) were sectioned in halves upon which thick (60-grit SiC) and thin (600-grit SiC) smear layer surfaces were produced. The adhesive application was randomized (n = 6) and a resin buildup was made. After 24 h, the resin-dentin sticks (0.8 mm2) were prepared. The GW on the sticks was measured at 400X magnification before the microtensile test (0.5 mm/min). The microtensile bond strength index was calculated including the relative failure type contribution. Data were analyzed by a two-way ANOVA and Tukey's multiple comparison. Simple linear regression was performed to determine whether correlation existed between µBS and pH, GW and pH, and µBS and GW. Only the factor "adhesive" significantly effected µBS (p = 0.001). TY had the lowest µBS values. Regarding GW, both main factors adhesive (p = 0.0001) and smear layer thickness (p = 0.03) were significant. The highest mean gap width was observed for SE and under a thick smear layer. A linear relationship was observed between the pH and GW and pH and µBS; however, a significant positive correlation was detected only between pH and GW (R = 0.99, p = 0.02). No relationship between µBS and GW was found (R = 0.04, p = 0.77). Although the smear layer thickness had no effect on resin-dentin bond strength, thick smear layers adversely affected the mean gap width. The µBS values were not affected by the acidity of self-etching primers; however, more aggressive self-etching primers showed the lowest mean gap width. The etch-and-rinse adhesive systems showed higher µBS and lower mean gap widths. Keywords: smear layer thickness, pH, microtensile bond strength, gap, dentin, self-etching, etch and rinse, adhesive system