Dentin cavities, prepared in extracted human teeth, were treated with various proprietary dentin‐bonding agents and then filled with a light‐cured restorative resin for posterior use. All bonding agents were either treated in accordance with the manufacturers' instructions or combined with Gluma, which is an aqueous solution of glutaraldehyde and HEMA, a hydrophilic monomer. 10 min after polymerization, the width and the extent of the marginal contraction gap was measured approximately 0.1 mm below the free surface of the filling, using a light microscope. With nearly all dentin‐bonding agents, the marginal contraction gap could be significantly reduced if Gluma was used after conditioning of the dentin. The reason for this improvement may be that glutaraldehyde cross‐links the collagen fibers and thereby strengthens the organic part of the hybrid layer, however, other mechanisms might also play a role in the improvement found.