Purpose: To compare the ability of three packable resin-based composite (RBC) systems (ALERT, Sure-Fil, and Solitaire), placed and light-cured in either incremental layers or in bulk, to seal the gingival margin of Class II preparations when the gingival margin is placed apical to the cementoenamel junction. Materials and Methods: Packable RBCs were used to restore 60 extracted human premolars with Class II preparations using an incremental-cure technique (each layer no greater than 2 mm in thickness when light-cured) or a bulk-cure technique (n=10). Manufacturers' directions were followed; the cure technique was the only variable. After the restorations were completed, the specimens were thermocycled, stained, sectioned, and viewed under a light microscope for microleakage at the gingival margin. Results: All specimens showed leakage at the gingival margin. No statistically significant difference was found between the bulk-cure and incremental-cure techniques for each of the RBC systems evaluated. ALERT had significantly less leakage than SureFil and Solitaire.