Research that has been recently carried out on t h e dielectric behaviour of conductor-insulator composite systems in the microwave domain has shown that the complex permivitty e* = e ' j e " depends strongly on the nature and the concentration of the conducting medium [1-4]. The dielectric permittivity of the samples has been calculated from the experimental determination of reflection and transmission coefficients of electromagnetic waves. The experimental values of complex permittivity in these media are in disagreement with the provisions of effective medium theory [5]. This is due to the fact that this theory does not take into account the formation of clusters of particles in contact which is revealed by d.c. electrical conductivity measurements [6-8]. This disagreement has also been observed for high volumetric concentrations of conducting medium in the visible and infrared regions [9-11]. Measurements performed as a function of sample thickness of carbon black (Monarch 700)--epoxy for different concentrations of conducting medium show that the measured reflection coefficient agrees with that calculated for an equivalent homogeneous medium. However, the calculated transmission coefficient differs significantly from the calculated value [12]. Such anomalies lead us to wonder about the validity of the concept of effective permittivity, which is supposed to represent, as is generally admitted, heterogeneous medium behaviour when particle size is much smaller than sample size and wavelength. The electrical properties of heterogeneous media, which are constituted of pure dielectrics or conductors, are perfectly defined by mixture formulae [13-16]. In the case of conductor-insulator composite systems, these properties are strongly related to the inclusion size and to their shape [17, 18]. Accordingly, propagation laws which intervene in diffraction of the incident wave by the medium, answer some of the questions relating to mixtures by defining a complex propagation constant but not an equivalent permittivity [19-21]. This work is particularly interested in the different