The fracture behavior of blends of nylon 6 and acrylonitrile-butadiene-styrene (ABS) compatibilized with an imidized acrylic (IA) polymer was examined by Izod impact testing and single-notch three-point bend (SEN3PB) instrumented Dynatup tests. The effects of the method of fracture surface measurement, ABS content, specimen thickness, compatibilizer content and fracture zone geometry were investigated. Blends containing a fixed (5 wt.% IA) compatibilizer content were tough over a broad range of ABS contents; the optimum toughness occurred near 50 wt.% ABS. A dual-mode of fracture was observed in SEN3PB specimens whose Izod impact samples with the same composition had ductile-brittle transition temperatures near room temperature. In these SEN3PB samples, ductile deformation occurred in samples with shorter ligament lengths, whereas brittle failure prevailed in samples with longer ligament lengths. The critical Ligament length at which the ductile-to-brittle transition occurs was shown to be dependent on the compatibilizer content and specimen thickness. These dual modes of fracture were rationalized in terms of a plane-strain to plane-stress transition. For blends that were super tough and had good low temperature toughness as judged by Izod impact testing, the toughness of SEN3PB specimens was generally insensitive to specimen thickness; these blends were fully ductile over the entire range of ligament lengths. The size of the stress-whitened zone was examined for fractured SEN3PB specimens that were fully ductile over the entire range of ligament lengths. Among specimens of a given composition, the size of the stress-whitened zones was geometrically similar and independent of the size of the original ligament. However, when ductile samples of different composition were compared, the size of the stress-whitened zone was not necessarily proportional to the energy dissipated during plastic deformation. This may be a result of the presence of different modes of energy absorption in the nylon 6 or SAN matrix phase.