Purpose: To evaluate the effect of additional acid etching on the microtensile bond strength (µTBS) and Weibull modulus (m) of two adhesive systems applied to caries-affected dentin. Materials and Methods: Ninety human molars with coronal carious lesions were sectioned to produce flat coronal surfaces containing caries-affected (CAD) and sound dentin (SD). The teeth were allocated to 6 groups: SB-CT: Control - Single Bond applied according to the manufacturer's instructions; SB+15: Single Bond with additional 35% phosphoric acid etching for 15 s; SB+30: Single Bond with additional etching for 30 s; CF-CT: Control - Clearfil SE Bond applied according to the manufacturer's instructions; CF+15: Clearfil SE Bond with additional 35% phosphoric acid etching for 15 s; CF+30: Clearfil SE Bond with additional etching for 30 s. Composite crowns were incrementally built up. The teeth were sectioned after 24 h in order to produce 2 to 3 slices involving each dentin substrate. The slices were trimmed into hourglass-shaped specimens (n = 30) and tested in tension at a crosshead speed of 1 mm/min. Data were analyzed by three-way ANOVA, Tukey's test (p 0.05) and Weibull statistics. Results: The µTBS to CAD was lower than to SD, except for SB+30. The additional etching increased the µTBS to CAD. The µTBS to SD increased with CF, but decreased with SB. CAD produced lower values of m with both adhesive systems. SB resulted in lower m in sound dentin than CF. The additional etching did not affect the m. Conclusion: The additional etching increases the µTBS to CAD, but not enough to reach the µTBS values obtained in SD. The structural reliability of the bond to CAD is not affected by the additional etching. Keywords: dental adhesive, demineralized dentin, microtensile bond strength test, resin-dentin interface, Weibull analysis