Objectives: Study the ability of adhesive materials to hybridize and seal root canal dentin. Methods: 36 maxillary incisors and canines were endodontically treated and randomly assigned to 4 groups: (1) AdheSE DC+Multicore Flow (ADH, Ivoclar Vivadent), a DC self-etch system; (2) Excite DSC+Multicore Flow (EXC, Ivoclar Vivadent), a DC total-etch system; (3) RealSeal (RS, Sybron Endo), a self-etch primer/sealant with Resilon points (Resilon Research); (4) Gutta-percha/AH26 (GP, Dentsply) as control. For each group, 3 teeth were assigned to one of 3 methods: (A) FESEM of the interface between root dentin and filling material (1mm thick slices); (B) TEM of the interface between root dentin and filling material; (3) SEM of fitting surface upon dissolution of the root in 6N HCl/48h and 2.5% NaOCl/15min. Results: (A) FESEM - the 3 adhesive materials formed hybrid layers (HL) but showed sporadic separation. GP showed wide areas of separation. (B) TEM - wide interfacial gaps for GP specimens, less frequent gaps for the other 3 materials; observed particles of RS of distinct shape/electrolucency. RS and ADH resulted in a partially calcified HL, while that of EXC contained no hydroxyapatite. Width of HL = 0.3-0.4µm for ADH, 0.5-0.6µm for RS, and 4.0-4.3µm for EXC. No HL was formed with GP; (C) The fitting surface of each material showed distinct patterns. For EXC, resin tags were observed at all levels, but less frequently in the apical third. ADH resulted in less abundant resin tags than EXC. The apical third of ADH did not display resin tags. For RS, resin tags were rare and virtually absent from the apical half of the root. Conclusions: In spite of the hybridization, a perfect seal of the root canal is difficult to achieve because of the complexity of the substrate. The microscopy methods used in this study complement each other.