The mechanism of resin adhesion to caries-affected dentin is still unclear. This study evaluated the interfacial morphology of two bonding systems to caries-affected dentin, coupled with the measurement of microtensile bond strengths (mu TBS). Carious human molars were prepared as previously described in Nakajima and others (1995) and were bonded with Single Bond (SB) or FluoroBond (FB) according to the manufacturer's instructions, followed by creation of AP-X composite buildups. After one day of storage in 37 degrees C water, the teeth were serially sectioned vertically into 0.8 mm slabs, trimmed to yield a 1 mm2 test area, and tested to failure in a Bencor device used in an Instron machine operated at 1 mm/min. Resin-dentin interfaces were observed with SEM before or after acid/base challenge. Bonding to normal dentin with the two bonding systems (SB and FB) showed tensile bond strengths significantly higher than those to caries-affected dentin. The moist bonding technique significantly increased bond strength of SB to normal and caries-affected dentin. SEM examination revealed that typical hybrid layer and resin tags could not be formed to caries-affected dentin. The results suggested that resin penetration may be prevented by occlusion of dentinal tubules by mineral deposits that may also impart acid-resistance to the intertubular matrix of caries-affected dentin.