Clays are ubiquitously located in the Earth’s near surface and have a high impact on the subsurface permeability. Most geo-electrical characterizations of clays do not take into account the heterogeneous nature of clay geological media. We want to better understand the influence of heterogeneities on the geo-electrical signature, thus we collected a dataset of spectral induced polarization (SIP) of artificial heterogeneous non-consolidated clay samples. The samples are made of illite and red montmorillonite in a parallel and perpendicular disposition (with respect to the applied electric field). Another sample is a homogeneous mixture composed of the same volumetric fraction of illite and red montmorillonite. For all the samples, the polarization is dominated by the red montmorillonite, given by the shape of the spectra (presence or lack of a peak at a particular frequency). We compared the experimental data with classical mixing laws and complex conductance network models to test how to better predict the SIP signature of such mixtures when the SIP spectra of the two components are known. The real conductivity is better predicted by the mixing laws, but the shape of the spectra (presence of polarization peaks at particular frequencies) is best predicted by the conductance network models. This study is a step forward towards a better characterization of heterogeneous clay systems using SIP.