Two Class II boxes of standarized dimensions were prepared in each of 10 extracted third molar teeth. The dentinal adhesives Syntac, Gluma 2000, Scotchbond MP, and All-Bond 2 were applied to five boxes each. All the boxes were restored with the same photopolymerizing resin composite. The microleakage of each restoration was measured ina permeability cell. The physiologic solution able to seep between the walls of the cavity and resin composite was measured and the flux of the liquid over time was calculated. All-Bond 2 was the only adhesive found, in some specimens, to make the dentin completely impermeable and to form an extremely precise seal between the resin composite material and the underlying dentinal surface. The flux values for all four adhesives were significantly different; Syntac allowed the greatest amount of microleakage. However, these in vitro results should not be considered absolute values.