Composite materials with varying toughness levels were tested using the Iosipescu shear test method. The failure modes were analyzed and classified into brittle, mixed or shear modes. It is shown that the tensile strength of the composites showing brittle failure can be determined directly from the load values obtained by the Iosipescu shear test. The advantages of the use of this test method in relationship to the common tensile or flexure test methods usually used to determine the tensile strength of brittle materials are discussed.