Viscosity, surface tension and contact angle are factors that influence the penetration of restorative resins into acid etched enamel. Furthermore, as the resin is drawn by the capillary forces into the pores of the etched enamel the pressure of the entrapped air will increase. The increased pressure has the effect that air will dissolve in a resin that is saturated with air at one atmosphere. The purpose of the present work was 1) to investigate the rate of dissolution of included air bubbles at increased pressure, 2) to use the results to calculate the depth of penetration by means of a cylindrical model of the capillary pores, and 3) to check the results of the calculations by measurements of the tag lengths of restorative resins placed on acid etched enamel. The rate of dissolution was measured in monomers of varying viscosity in a glass syringe by means of a stereo microscope. The calculations showed that the depth of penetration decreases only slightly with viscosity. Thin sections of restorative resins placed on acid etched enamel were prepared whereafter the enamel was dissolved in hydrochloric acid. Tag lengths of 50 micron or more were observed with composite as well as non-composite resins.