There were conducted four series of biharmonic experiments on cylindrical samples of circular section. Describes how to determine the dynamic mechanical properties of low-modulus viscoelastic composites at biharmonic law of loading. With this technique, the experimental data were processed. As a result, were received diagrams of dependences values of the angle of lag between stresses and deformations of high-frequency harmonics from changes values of deformations of low-frequency harmonic and diagrams of dependences values of the dynamic modulus of elasticity of high-frequency harmonics from changes values of deformations of low-frequency harmonic for different values of frequencies the low-frequency harmonics. It was a comparison of the dynamic mechanical properties at biharmonic law of loading with the results of the quasistationary dynamic experience.