This work discusses the development of and subsequent use of a novel method for the measurement of stability derivatives of models in a hypersonic gun tunnel using the free-flight technique. The technique uses a passive electro-optical system to illuminate and scan a rectangular field of view in the vertical plane through the test-section of the gun tunnel. 6 vertical laser sheets, produced by the expansion and reflection of 3 He-Ne laser sources, are used to backlight the model as it flies through the field of view. Shadows from sheets interrupted by the model are projected onto rapid-access photo-sensitive arrays. The position and pitch angle of a model traversing the field of view can be determined by a knowledge of the recorded shadow positions and the external geometry. The non-linear differential equations of motion are fitted to the resulting motions using a parametric differentiation technique embedded in a modified least-squares minimisation scheme. Convergence of the minimisation procedure yields the stability terms which are of interest. The effects of model geometry and mass distribution on the resulting free-flight static and dynamic stability are assessed and discussed.