The capability of dental restorative materials to effectively seal the margins of tooth preparations is critical. The adhesive properties of the conventional chemically-cured glass ionomer cements have provided a potential for sealing cavity margins, but recently light-cured versions of the glass ionomer cements have been introduced. This study examined the adhesion to dentin of two new light-cured glass ionomer cement restorative materials, Fuji II LC and VariGlass VLC. Gap formation at the tooth/restoration interface in dentin cavities was recorded as a measure of the sealing ability of the materials. Cylindrical cavities were prepared in dentin of extracted teeth and randomly assigned to three equal groups. The control group of specimens was restored with a chemically-cured glass ionomer, Fuji Cap II, while the experimental groups were restored with one of the two light-cured materials. All specimens were subjected to thermocycling. The sealing of each material to dentin was evaluated by measuring the gap between the restoration and tooth structure. The control group registered mean gaps of 26 microns while the experimental groups registered mean gaps of 8 microns (Fuji II LC) and 10 microns (VariGlass VLC). The light-cured materials displayed substantially better adaptation to the cavity preparations than the conventional materials (P < 0.005), with no significant difference between the two experimental groups.