The shear fracture strength and the critical stress intensity factor in mode II loading of lactose monohydrate and acetylsalicylic acid powder compacts has been evaluated. The experimental results of the shear fracture strength and the critical stress intensity factor in mode II loading appeared to be in good agreement with powder behaviour such as lamination and capping during compaction. Values for the critical stress intensity factor in mode II loading depended on the depth of the crack and hence, any reference of such values or their use to calculate a "fracture toughness ratio" (K-IC(I)/K-IC(II)) must refer to the notch depth applied. The results confirmed that the failure of such powder compacts occurs mainly in tension, but that lactose monohydrate has a tendency also to fail in shear. The latter does not apply to acetylsalicylic acid. Hence, lactose monohydrate should only be used cautiously in layer or press-coated tablets.