In the past, dentinal adhesives were evaluated in Class V or cylindrical dentinal cavities. There have been relatively few reports on the effectiveness of dentinal adhesives in posterior restorations. In the present study, five of the most recently developed dentinal adhesives were examined to determine their ability to seal margins. None of the dentinal adhesive and posterior composite resin combinations demonstrated a complete seal at the proximal cervical margins of Class II mesio-occlusodistal restorations. In general, margins finished in enamel were sealed against leakage better than were those finished in dentin. A glass-ionomer lining cement did not eliminate the marginal leakage of Class II composite resin restorations.