Background and Objective: Polymerization shrinkage in composite resins is responsible for microleakage restoration. A recently introduced composite resin Filtek P90 is based on siloxanes and oxiranes which polymerize by cationic "ring opening" polymerization resulting in reduced polymerization shrinkage. The aim of this study was comparative evaluation of microleakage in class II cavities restored with a silorane resin composite (Filtek P90) or a methacrylate based composite (Z250) using two different techniques of total bonding and open sandwich. Materials and Methods: A class II cavity was prepared on the mesial and distal surfaces of twenty four caries-free premolars, with the gingival margin of the cavity being 1 mm under CEJ. Teeth were assigned to restorative material and operative techniques (total bonding, open Sandwich (resin composite, RMGI), to four groups of 12 cavities: I: silorane-based composite (Filtek P90) II: RMGI (Fuji II) & Filtek p90 III: methacrylate based composite (Z250) IV: RMGI & Z250. The restored teeth were examined for microleakage after thermocycling) 5-55°c) and immersion, in a basic fuchin 0.5% solution (24 hours) and sectioned with diamond disk, using stereomicroscope (20X). The degree of dye penetration was recorded and analyzed with SPSS16 and Fisher’s exact test. Results: Differences between group I with other groups were significant (P<0.01). Group III showed most microleakage in the degree of dye penetration 2. No significant difference was observed in groups II, I and IV. Conclusion: Open sandwich technique (groups I, IV) and p90 composite (total bonding) in group I showed the least microleakage. References 1- Türkün LS, Aktener BO, Ateş M. 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