This part of the International Standard IS0 6721 describes an ultrasonic wave propagation method for determining the storage components of the longitudinal complex modulus L* and the shear complex modulus G* of polymers at discrete frequencies typically in the range 0.5 MHz to 5 MHz. The method is suitable for measuring materials with storage moduli in the range 0.01 GPa to 200 GPa and with loss factors below 0.1 at around 1 MHz. With materials that have a higher loss, significant errors in velocity measurement are introduced through waveform distortion and can only be reduced using procedures that are outside the scope of this standard. The method allows measurements to be made on small specimens, typically 50 mm x 20 mm x 5 mm, or small regions of larger specimens or sheets. It is therefore possible to obtain information on the homogeneity or anisotropy (see clause 10.5) of modulus in a specimen.