In this work results on the preparation, characterization and single cell testing of matrices for phosphoric acid fuel cells are reported. The matrices were produced using mixtures of powders of silicon carbide with zirconium silicate or niobium carbide, by deposition of an aqueous emulsion of the powders on top of the catalyst layer of the gas diffusion electrodes, or by making a self-supported him of the materials. The physical characterization of the powders and of the matrices included the distribution of particle size and shape of the powders and the thickness, electrical resistance, and pore size distribution in the matrices. The matrices were evaluated under operation, using standard supported platinum on carbon gas diffusion electrodes in a H-2/O-2 Single fuel cell setup operating with phosphoric acid at 180 degrees C. The results of the cell potential-current density characteristics were analysed to separate the contribution of the structural properties of the matrices on the total polarization losses of the single cells. It was found that a single cell operating with a self-supported matrix composed of 50 wt% NbC + SiC has a much better performance than that observed with SiC alone.