The daily use of composite resins, the rapid evolution in their composition and that of adhesives, as well as the defects in their dimensional stability on setting, led us to carry out this study on the sealing ability of modern bonded aesthetic fillings by measurement of the thickness of the dentine-filling hiatus. In vitro, category V dental fillings are made then sectioned, polished, dyed and observed under optic microscopy. Penetration of the dyeing agent unto the outer two-thirds of the fillings proves the lack of tightness. Enamel-composite limits are almost always acceptable. The dentine-composite hiatus measures between 4 and 8 micrometers on average with the use of new adhesives (from 1 to 10 micrometers maximum). This represents a substantial improvement in comparison to the old generation of adhesives which allowed hiatuses of 10 to 50 micrometers to show. No particular product of the new generation showed to be better in this study.