This paper discusses an investigation into the applicability of torsion of filament-wound tubes to determine the in-plane shear properties of ±45° Kevlar and S-glass/epoxy com-posites. Results show that this test method can be used to determine in-plane shear properties of these materials, although failure by buckling must be prevented if valid in-plane shear strength values are to be obtained. The Kevlar/epoxy fails under in-plane shear by compressive microfrac-ture of the compressively loaded -45° regions of the tube at about half the strength of the S-glass/ epoxy. This failure appears as a yield point in the stress-strain curve of the Kevlar/epoxy, and is accompanied by a largely irrecoverable drop of more than an order of magnitude in the shear moedulus. The results also indicate that the in-plane shear strength of a filament-wound composite depends upon its winding pattern, with the use of tighter crossover spacing producing a lower strength composite.