The effect of the phase angle psi on fracture toughness and fracture mechanisms of planar interfaces between polystyrene (PS) and poly(2-vinylpyridine) (PVP) reinforced with deuterium-labeled polystyrene (dPS)/PVP block copolymers was investigated using an asymmetric double cantilever beam (ADCB) specimen. The fracture toughness of an interfacial crack, G(c), was measured as a function of the phase angle psi, the areal chain density of the block copolymer SIGMA, and the degree of polymerization N of the chains. The fracture mechanisms of the interface were studied by forward recoil spectrometry (FRES), which permitted the location of the dPS block to be determined after fracture. There is a dramatic change in fracture toughness as the phase angle changes sign, a change which is associated with the formation of crazes in the PS at, and away from, the interface. The angle theta of craze propagation away from the interface depends on the phase angle psi. For positive phase angles, theta is close to 45-degrees; for negative phase angles, theta is close to 135-degrees.