The influence of specimen thickness on the fracture behaviour of polycarbonate (PC) and co-continuous PC/ABS (50150) blends was studied in single edge notch tensile tests at 1 m/s and different temperatures (-80 to 130 degreesC). Specimen thickness ranged from 0.1 to 8 mm. In the co-continuous PC/ABS blends the rubber concentration in the ABS was 0, 15 and 30 wt%. The change in fracture toughness was typified by the change in brittle-to-ductile transition temperature (T-bd). T-bd of pure PC depended strongly on specimen thickness, leading to very low transition temperatures for thin PC specimens. PC/ABS 0%, a 50/50 blend of PC and SAN (i.e. ABS without polybutadiene (PB)), was a brittle blend and showed a very high T-bd close to the T-g of SAN. T-bd did not seem to be influenced by specimen thickness. PC/ABS blends with 15 and 30% PB in ABS showed improved T-bd compared to PC/SAN and PC, indicating effective rubber toughening. T-bd decreased with decreasing thickness for PC/ABS specimens thicker than 1.5 mm. However, T-bd increased with decreasing thickness for specimens below 1.5 mm thickness. In thin specimens, the rubber-filled blend is less effective rubber toughening. The plane strain stress condition needed for rubber cavitation is apparently not present in thin specimens.