The development of novel Bis-GMA derivatives incorporated as base monomers in resin-based composites (RBCs) may offer a potential reduction in polymerization stresses generated in the tooth/restoration complex compared with conventional Bis-GMA. The hypothesis of the current study was that RBCs based on either a urethane derivative of Bis-GMA (Bis-GMA1) or a Bis-GMA superior oligomer (Bis-GMA2) would produce less cuspal deformation and less cervical enamel microleakage than RBCs based on a conventional Bis-GMA. Standardized MOD cavities were prepared in extracted upper premolars (n=9) and restored with each of the three experimental RBCs. The cuspal deformation of premolars was recorded during and post-irradiation using direct current differential transformer (DCDT) displacement transducers, and data were analyzed using Kruskal-Wallis and Mann-Whitney tests at a p0.05 level of significance. The polymerization process of the three RBCs determined cuspal extension during exposure to irradiation, followed by cuspal deflection when the light source was turned off. Both composites containing Bis-GMA derivative monomers were determined to result in significantly reduced cuspal extension compared to the composite based on Bis-GMA. Only the composite based on Bis-GMA2 monomer generated a significantly reduced cuspal deflection compared to the composite based on Bis-GMA. No dye penetration was detected at the cervical enamel margin of any restoration.