The principle of the method specified is subjecting a standard test specimen to a bending stress to produce one of the nominal surface stresses (method A: 1,80 MPa, method B: 0,45 MPa, method C: 8,00 MPa), raising the temperature at a uniform rate, measuring the temperature, at which a specified deflection occurs. The test specimens are tested in one of two positions, flatwise or edgewise, the requirements on test-specimen dimensions being different in each case. The methods give better reproducibility with amorphous plastics than with semi-crystalline ones.