We studied the fracture behavior of trilayer A/B/A assemblies based on polystyrene (PS) and poly(methylmethacrylate) (PMMA) where the central layer of the A polymer was confined (0.5-200 mu m) between two thick plates of the B polymer (1- 3 mm). Diblock and random P(S-MMA) copolymers were used to provide a good stress transfer across the interfaces. Fracture experiments were performed with the double-cantilever beam method and the fracture mechanisms were observed by optical microscopy on microtomed slices of the: damaged zone. The measured G(c) of the A/B interface fractured during the test was dependent on the molecular structure at the interface (random copolymer, diblock copolymer or no copolymer), on the crazing stress of the hulk materials and on the interfacial sheer stresses. When the phase angle of the loading was even slightly positive, oblique crazes were observed in the PS increasing greatly G(c). If PS was the central layer, this resulted in a very marked dependence of G(c) on the thickness of the central layer for a thickness range 10-200 mu m which was not observed when the PMMA was the central layer. Thermal treatments modifying the interfacial shear stresses were also found to have a very strong effect on G(c).