Binary blends of unbranched polyethylene (PE) and 5-10% model ethylene-butene random copolymers are used to determine the effects of composition heterogeneity on phase separation in the melt, semicrystalline morphology, plane strain fracture toughness J(C) and tensile modulus and yield strength. Slowly cooled samples of melt-miscible blends are appreciably tougher (J(C) = 5.2 kJ/m2) than unblended PE (J(C) = 2.7 kJ/m2). A blend with the same average short chain branch concentration, but which is phase separated in the melt state, has J(C) = 3.3 kJ/m2; dispersed domains of amorphous polymer have little effect on toughness. Enhanced toughness is associated with nonuniform morphology formed on slow cooling "one phase" melts composed of chains with different amounts of branching. The relative number of chemically different chains, as opposed to absolute branch concentrations, seems most important. Tensile properties are relatively unaffected by blending at these levels. Results from these model blends are used to consider the properties of compositionally heterogeneous ethylene copolymers.