The present study investigated the effectiveness of treating dentin with 2-hydroxyethyl methacrylate (HEMA) prior to application of an adhesive resin. The adhesive resin was 5% 4-methacryloxyethyl trimellitate anhydride (4-META) in methyl methacrylate (MMA) combined with poly-MMA powder. Polymerization of this resin was initiated by tri-n-butyl borane (TBB). Bovine dentin samples were ground with 600-grit Carbimet paper discs, and demineralized with either an aqueous solution of 10% citric acid/3% ferric chloride (10-3) or an aqueous solution of 10% citric acid (10-0). Improved bond strengths were achieved with HEMA treatment of bovine dentin samples, and improvement of bond strengths was dependent upon the time period of HEMA application. Scanning electron microscope (SEM) examination revealed the formation of a transitional zone of resin-reinforced dentin, termed the "hybrid" layer, in those specimens receiving 10-3 pre-treatment. The adhesive resin impregnated the exposed collagen bundles with which it entangled to create the "hybrid", essential in the attainment of high tensile bond strengths. Specimens pre-treated with 10-0 did not readily form "hybrid" layers. However, if HEMA application followed the 10-0 pre-treatment, "hybrids" were demonstrated on SEM, and bond strength increased to 13 MPa. The ferric ions in the 10-3 effectively improved the diffusivity of dentinal substrates, as did HEMA. This study indicates that HEMA applied to dentinal substrates enhances monomer diffusion and entanglement with dentinal components, and facilitates the formation of a "hybrid" layer.