A new method, based on tensile yield strength and strain, has been developed to normalize the effect of matrix properties on the critical fibre length and the interfacial shear strength obtained from the fragmentation test. It is argued that the conventional data normalization technique which employs elastic properties of the matrix, is fundamentally flawed because the model employed to calculate interfacial shear strength assumes perfect plasticity. Single embedded fibre fragmentation in a range of epoxy resins with differing mechanical properties has been used to validate the new method. Stoichiometric quantities of the current agent were used to keep the same interfacial chemistry. The proposed method provides more consistent interfacial shear strength data than existing theories. Furthermore, this normalization technique can also be used to predict the interfacial shear strength of glass fibres embedded in a range of support resins, such as vinyl ester or epoxy resins. For these cases, a thin layer of the phenolic resin was used on the glass fibre to keep the interface chemistry the same.