PURPOSE: To investigate the morphology of the resin-dentin interface of three dentin adhesive systems (One Coat Bond, Clearfil SE Bond and One-Up Bond F) bonded to caries-affected dentin, exposed using either rotary instrumentation in conjunction with a caries detector dye or a chemo-mechanical caries removal system (Carisolv). MATERIALS AND METHODS: 60 extracted human molars with occlusal caries were used. 30 teeth had carious dentin removed using slow-speed round steel burs after staining with the caries detector dye, and carious dentin from the remaining teeth was removed using Carisolv. The teeth from each caries removal technique were randomly divided into three groups, bonded with one of the three adhesive systems above according to the manufacturers' instructions, and a thin layer of resin-based composite applied. The teeth were sectioned parallel to the bonded surface to obtain a 3 mm thick bonded dentin disc. The specimens were assigned to one of two observation techniques: a fracture technique or an acid-base technique. Fracture technique: a shallow groove was prepared across the dentin surface of the specimen, fixed in 10% buffered formalin, dehydrated in an ascending ethanol series up to 100%, critical-point dried, and fractured along the prepared groove. Acid-base technique: the specimens were sectioned vertically across the bonded interface, fixed in 10% phosphate buffered formalin, polished with diamond paste down to 0.25-microm particle size, immersed in 10% orthophosphoric acid for 3-5 s and 5% sodium hypochlorite for 10 min. All specimens were gold sputter-coated and observed using FE-SEM. RESULTS: All groups showed hybrid layer formation. However, the thickness varied depending on the adhesive system and the carious dentin removal technique used. One Coat Bond (a "one-bottle" system) showed the thickest hybrid layer (approximately 2 microm) in acid/base treated specimens when bonded to caries-affected dentin, whereas One-Up Bond F (an "all-in-one" system) bonded to normal dentin exhibited the thinnest hybrid layer (approximately 0.3 microm).