This study reports the effects of five commonly used bases on the hardness of three newer composite resins. The bases were a calcium hydroxide, a polycarboxylate, a ZOE, an EBA, and a glass ionomer cement. All bases except the calcium hydroxide caused reduction in the hardness of each composite, although for some base/composite combinations the reductions were not significant. Glass ionomer also caused considerable reduction in surface hardness and cannot be recommended for use as a base under composite resin restorations.