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Based on experimental and numerical investigations minimum size specimenrequirements are proposed which allow for the determination of valid shear mode-IIfracture toughnesses KIIC. Following, in principle, the methodology for determining themode-I fracture toughness KIC, the established requirements ensure a dominating planestrain state of stress at the crack tip and plastic zones that are sufficiently small withrespect to the specimen dimensions to allow for applying a linear-elastic approach. Therequirements have been established for specimens used in Arcan-Richard type loadingfixtures which allow testing under different ratios of mode mixity. A comparison ofcharacteristics of the crack tip stress field obtained under mode-I and mode-II conditions ofloading with A-R-specimens is made with results obtained with SEB- and CT-specimens formeasuring mode-I fracture toughnesses KIC according to ASTM E 399. The studydemonstrates that the minimum specimen thickness for KIIC-tests can be smaller, but thatthe minimum dimensions of the specimens in in-plane directions should be larger than for aKIC-test.1.