This study investigated the effect of different cavity lining techniques on the marginal sealing of Class II composite restorations. A total of 36 human molar teeth, free of caries and fillings, were each prepared with two proximal Class II boxes mesially and distally. In mesial boxes, cavity liners were applied as follows: in group 1, separately cured flowable composite; in group 2, co-cured flowable composite; and in group 3, resin-modified glass ionomer (RMGI). The remaining cavities were filled incrementally with a universal restorative composite. The distal boxes were filled with no liner as controls. After thermocycling, the specimens were immersed in a silver nitrate solution and the microleakage was evaluated. Analysis of variance showed that the degree of microleakage for group 3 was significantly lower than that of the other groups. Based on the results, it was concluded that the use of RMGI as a cavity liner under composite restorations showed the least microleakage. Flowable composites, whether co-cured or separately cured, had no influence on the marginal sealing.