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Resin–dentin bonds are known to degrade in the relatively aggressive oral environment. In order to obtain greater insight into the interfacial degradation process, we examined, by using transmission electron microscopy (TEM), the interfacial ultrastructure of two adhesives bonded to dentin after 1 yr in vivo. Class V cavities were prepared on the buccal surfaces of 14 intact teeth of two monkeys and then restored by using either the two-step self-etch adhesive, Unifil Bond, or the two-step etch-and-rinse adhesive, Single Bond, in combination with the restorative microhybrid composite, Z250. After 1 yr, 10 other teeth were restored by using the same materials (controls). One day later, the monkeys were killed, following which the microtensile bond strength to dentin was determined and the interfacial ultrastructure was examined by TEM. Whereas no noticeable changes in the morphology of the resin–dentin interface were observed between the 1-d and 1-yr specimens for Unifil Bond, Single Bond exhibited signs of interfacial degradation, in particular at the bottom of the 3 µm-deep hybrid layer. In conclusion, the adhesive interface produced by the etch-and-rinse adhesive was less resistant to degradation than that produced by the self-etch system. [ABSTRACT FROM AUTHOR]