The Adhesion-Decalcification concept (Yoshida et al., JDR 2001) dictates how molecules interact with hydroxyapatite-based tissues. Objectives: To revisit the AD-concept as thought to be fundamental for durable bonding, and to explore why ‘mild' self-etch adhesives less effectively bond to enamel than dentin. Methods: The interaction of three monomers (4-MET/10-MDP/phenyl-P) with hydroxyapatite (HAp), and for an experimental 10-MDP:EtOH:H2O (15:45:40wt%) self-etch primer also with dentin/enamel, was characterized by XRD, and further complemented with TEM-interfacial ultra-structural data upon its reaction with dentin. Finally, the tensile bond strength (TBS) to enamel/dentin of the primer (combined with Clearfil SE ‘Bond') with and without 1wt% camphorquinone (CQ) added, was measured [Clearfil SE (C-SE, Kuraray) served as control]. Results: Regarding molecular interaction, the two extremes were phenyl-P that rather ‘etched' and 10-MDP that rather ‘bonded' to HAp (with 4-MET behaving somewhat in between). ‘Etching' was associated with more substantial/rapid DCPD (CaHPO4•2H2O) deposition, while ‘bonding' was represented by the formation of a Ca-monomer nano-layered structure (confirmed by TEM). XRD revealed a higher crystallite size/cristallinity for enamel, while nano-layering was more pronounced at dentin. Nano-layering was enhanced by actively rubbing the surface with the exp. primer for 20sec (than when enamel/dentin was solely immersed for 30min in 5ml primer). The exp. primer needed CQ to obtain a TBS to dentin comparable to that of C-SE, but not when bonded to enamel. Conclusion: Stable ionic-bond formation to HAp competes in time with the deposition of less stable calcium-phosphate salt deposition (DCPD). For durable bonding, Ca-monomer salt formation should precede/exceed DCPD deposition. The lower bonding effectiveness of mild self-etch adhesives to enamel should be ascribed to a lower chemical reactivity (nano-layering) with enamel-HAp, in combination with less potential for micro-mechanical interlocking (as for instance achieved by phosphoric-acid etching). Adequate polymerization (by CQ) is needed to stabilize the formed nano-layer.