Objective: To assess the marginal adaptation of ormocer-, silorane-, and methacrylate-based composite restorative systems bonded to dentin cavities after water storage. Method and Materials: Cylindric dentin cavities were prepared on the buccal surfaces of 45 extracted human molars after their enamel was removed. The cavities were assigned to one of three groups (each n = 15). Each group was restored with one of three composite/adhesive systems: an ormocer-based (Admira/Admira Bond), a silorane-based (Filtek P90/P90 System Adhesive), and a methacrylate-based (Tetric Ceram/Excite). Marginal adaptation was evaluated immediately after polymerization and after 1 month and 1 year of water aging and thermocycling. Evaluation was performed under a metallographic microscope by recording frequency of gap-free restorations, width of maximum marginal gap (MG), percentage length of debonded margins relative to cavity periphery (DM), and marginal index (MI = MG × DM/100). The results were statistically analyzed with two-way ANOVA and the Tukey test at α = .05. Results: None of the composite/adhesive systems examined exhibited gap-free restorations at any aging time. Marginal adaptation was significantly affected by the type of restorative/adhesive system, while water aging time had no significant effect. Silorane composite restorations exhibited the lowest MG, DM, and MI values, while methacrylate composite restorations revealed the highest values. Ormocer restorations showed intermediate values. Conclusion: Although all examined composite systems failed to achieve gap-free margins with dentin cavities, the silorane-based system revealed the best marginal adaptation at all aging times. Keywords: composite resin, dentin bonding, marginal adaptation, ormocer, silorane