A method is proposed to identify the mechanical properties of the skin and core materials of sandwich structures having heterogeneous cores. All the elastic coefficients and loss-factors that matter in the dynamics of such a panel in the thick-plate approximation are identified. To this end, experimental natural modes (i.e. eigenmodes of the damped system) are compared to the numerical modes of large sandwich panels (lx,y/h ≃ 80). The chosen generic model for the visco-elastic behaviour of the materials is E(1 + j ). The numerical modes are computed by means of a Rayleigh-Ritz procedure and their dampings are predicted according to the visco-elastic model. The frequencies and dampings of the natural modes of the panel are estimated experimentally by means of a high-resolution modal analysis technique. An optimisation procedure yields the desired coefficients. A sensitivity analysis assess the reliability of the method. Identification is conducted on two very different kind of sandwich panels to illustrate the method.