A simple test method for the determination of the temperature at which the transition from the 'glassy' to the 'rubbery' state occurs in stress-birefringent materials is described and illustrated. The method is intended primarily for materials used in 'frozen-stress' photoelastic studies; the temperature obtained is that at which the 'stress-freezing' process is complete. The birefringence in a small sealed cylinder of the material is measured after cooling the cylinder from above the transition temperature. Test instrumentation is minimal. Corresponding approximate values of Young's modulus and Poisson's ratio are also obtained from the cylindrical test piece.