The gap-reducing efficacy of four phosphate-based dentin-bonding agents was tested after various treatments of the dentin and/or the bonding agents. The investigation was carried out on extracted human teeth. One of the root surfaces was ground flat, and a cylindrical butt-joint cavity was prepared in the ground dentin surface. It was found that the efficacy of two of the adhesives, presumably being chlorosubstituted phosphates could be markedly improved if the primed dentin was rinsed with copious amounts of water, dried by compressed air, and a second layer of bonding agent applied. The mechanism behind this improvement is assumed to be the formation of hydrogen chloride, which dissolves part of the smear layer.