Oxalic acid is used to block tubule fluid flow when bonding to dentin. Objective: The aim of this project is to characterize the long-term ultra-morphology of resin-dentin interfaces, both with and without the use of oxalic acid and a total-etch, three-step adhesive system, (Optibond FL = FL) by examining nano-leakage. Methods: Freshly extracted human third molars stored in 0.5% chloramine were sectioned to expose dentin with a diamond disk and sanded with 600 grit silicon carbide paper to produce a uniform smear layer. Half of the dentin surfaces were treated with oxalic acid. Resin composite was bonded using FL, applied according to manufacturer's instructions. The teeth were sectioned occluso-gingivally into 0.5-mm-thick serial sticks. Eight sticks from each tooth, four treated with oxalic acid and four controls were obtained. After 48 hours, two control and two treated sticks from each tooth were treated with ammoniacal silver nitrate then processed according to conventional methods for TEM using Spurr's resin. The remaining four sticks were stored in artificial saliva at neutral pH for 4 months then prepared identically for TEM examination at various magnifications. Comparisons were made between the extents of silver ion penetration into the hybrid layer for all groups. Results: All groups showed silver ion penetration throughout the hybrid layer. However, the hybrid layer of the specimens treated with oxalic acids showed less silver ion penetration within the hybrid layer at 48 hours and 4 months. Conclusion: It seems oxalic acid applied to dentin before bonding reduces the amount of nano-leakage in the hybrid layer.

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