Purpose: This study examined the effect of dentin pretreatment with acetone or ethanol on bonding of one-bottle self-etching vs two-step self-etching adhesives to evaluate the effect of hydration on bond strength. Materials and Methods: The bond strengths of resins were measured using two one-bottle and two two-step selfetching adhesives that bonded to wet dentin (control), or acetone- or ethanol-pretreated dentin. Wet dentin is the normal substrate for bond testing, and in the pretreatment groups, 95% acetone or ethanol was applied to the dentin surface for 30 s before bonding. Weight of resin adhesives before and after evaporation was measured to evaluate the extent of the solvent contents in the adhesives. Furthermore, phase separation of adhesives was morophologically investigated by in situ light-microscopic observation to evaluate the hydrophilicity of the adhesive resins. Results: The bond strengths of the acetone and ethanol groups for one-bottle adhesives were significantly greater than for the control (p 0.05). However, significantly lower bond strengths were found in the pretreatment groups for the two-step self-etching adhesives (p 0.05). Conclusion: The bond strength depended on the wetness of the bonding substrate for both adhesive groups, with a positive effect in bond strength for one-bottle adhesives and a negative effect for two-bottle self-etching adhesives. The acetone- or ethanol-pretreated dentin surfaces may provide an improved substrate for resin adhesion of hydrophilic self-etching adhesives such as one-bottle self-etching adhesives due to dentin dehydration. Keywords: self-etching adhesive, acetone, ethanol, water, pretreatment, bond strength, phase separation, dehydration