Abstract Several composite restorative resins have recently been introduced for use in posterior teeth. A number of dentin bonding agents are commercially available for use in conjunction with composite restorative resins. The objective of this in vitro study was to evaluate the microleakage of Class II MOD preparations in extracted human mandibular molar teeth restored with a chemically-activated posterior composite resin (P 10) and a visible light-cured composite resin (P 30) used in conjunction with a low viscosity bonding resin (Enamel Bond) and a dentin bonding resin (Scotchbond), respectively. The restored teeth were subjected to 100 complete temperature cycles in an aqueous 45CaCl 2 solution between 5°C and 55°C and the microleakage evaluated from autoradiographs. Microleakage was observed at the cervical aspects of all the restorations. The results of this in vitro study suggest that microleakage may occur at these sites of posterior composite restorations.