The hypothesis tested in this study was that the self-etching system (Clearfil SE Bond, CSE) is less sensitive to surface moisture variability than the system that uses a separate acid-etching step (Single Bond, SB). Eighteen dentin specimens were bonded to composite using CSE or SB. Three different surface moisture conditions per bonding type (overwet, w; dry, d; and visibly moist, n [normal]) were applied prior to bonding dentin to composite. One cross section of each sample was analyzed with lines of nanoindentations crossing perpendicular to the bonding interface. An additional set of bonded samples was fixed and cross sectioned before the hybrid layer thickness was measured in scanning electron microscopy. The nanoindentations revealed significant differences in indentation modulus (E(i)) and hardness (H) for the hybrid layer comparing SBn, E(i) = 2.7(+/-1.6); H = 0.24(+/-0.1) GPa with SBd, E(i) = 0.9(+/-0.7); H = 0.9(+/-0.05) GPa, respectively, while CSE showed no differences among the groups. A significantly greater demineralized zone below the hybrid layer was found for SBd. The hybrid layer was wider for both CSEd and SBd. In conclusion the hypothesis was verified; CSE exhibited no significant changes of hybrid layer properties (E(i), H) at different hydration conditions, while SB had significant differences, especially after air-drying.