Examination of the K Ic to Young's elastic modulus relationship for structurally similar homogeneous glasses reveals that the toughness is directly proportional to Young's elastic modulus. This can be substantiated theoretically on the basis of an elastic fracture model. If, however, the results of K Ic measurements are examined for glasses known to be phase separated in the PbOB 2O 3 and Na 2OB 2O 3 systems, then a different fracture toughness to Young's elastic modulus relationship results. Higher Young's modulus phase separated glasses have the lower fracture toughnesses. This trend can be explained on the basis of microcracking in the crack tip process zone which shields the primary crack from the applied stress.