An experimental method is described which allows the shear modulus and Poisson’s ratio of a polymer material to be determined as functions of temperature. Using commercial thermal mechanical analysis equipment, measurements are required of the thermal expansion coefficients of both unconstrained and constrained specimens, and of the Young’s modulus. The constraint on a small disc-shaped polymer specimen is applied by a copper annulus. The basis of the method is that the apparent thermal expansion of the constrained specimen depends on the Poisson effect, and an analysis is given which enbles the Poisson’s ratio to be determined. Some illustrative results are presented for a silicon rubber.