The molecular basis of fracture of polystyrene and poly(methyl methacrylate) homopolymers and their latex blends was investigated with a custom-built dental burr grinding instrument (DBGI). About a third of the chains were cut several times, the remainder not at all. The number of chain scissions in polystyrene and poly(methyl methacrylate) was quantitatively interpreted by the microscopic parameters of craze fibrils and the energy balance between chain scission and chain disentanglement (chain pullout). The probability that a polymer strand in the craze fibrils is scissioned or disentangled was calculated from the fracture energy balance. In addition, the fracture energy of the latex blends of polystyrene and poly(methyl methacrylate) was studied. The large interface between the polystyrene and the poly(methyl methacrylate) did not lead to a small fracture energy, as initially expected. Rather, the latex blend of the two immiscible polymers primarily absorbs the fracture stress by strong co-continuous bulk phases.