Results of numerical simulations of the dielectric properties of binary composites with three-dimensional structures are verified by means of comparison with experimentally obtained data. Two different heterogeneous structures - regular (periodic) and stochastic, were studied. Frequency dependencies of the complex dielectric permittivities of the materials of the constituent phases of the composites were measured and were used as input for computer modelling together with their known geometrical parameters. Calculations were based on the effective media approach, and the associated field problem was solved numerically using the finite-volume method and the finite-element method. The results of the measurements and the computations of the effective complex permittivities of the composite materials are compared and are analysed.