The formation of mixed molecular layers is technologically important but the understanding of such molecular systems is limited. The comparative analysis of photoemission experimental results near threshold from organic dye layers, both in pure form and with a second kind of molecules present, shows that the energy levels of the dye molecules are affected by the presence of the foreign species. A theoretical model is presented for the calculation of the observed shift of the energy levels from Coulomb interactions in mixed aggregates and the results are applied to the study of the mechanism of supersensitization.