The fracture behavior for the partially miscible polystyrene/poly (styrene-r-4-bromo-styrene) (PS/PBS) interface is analyzed as a function of miscibility. This is achieved by measuring the fracture energy at PS/PBS interfaces over a range of molecular weights (1370 < N < 7144; N is the degree of polymerization) and annealing conditions (150 < T < 200degreesC) at varying degrees of miscibility (controlled by f, the volume fraction of brominated repeat units in PBS). Correlations between fracture energy, mutual diffusion coefficient (measured using Rutherford backscattering spectroscopy), and the Flory-Huggins interaction parameter (chi) are presented. It is shown that the fracture energy, G(c), increases as annealing temperature increases and N and f decrease. The presence of two miscibility-mediated interdiffusion/fracture regimes is postulated depending upon the value of N*chi, where N* = 2N(PS) N-PBS / N-PS + N-PBS. (I) When N*chi < 2, the system is miscible, G(c) is high, and the fracture mechanism is crazing; (II) When N*chi > 2, the system exhibits a miscibility gap, interdiffusion is thermodynamically limited, which results in a decreased G(c), which continues to decrease with increasing f. No crazes are observed in regime II and fracture occurs via chain pullout.