This paper studies polymer composites formed by a polystyrene matrix (PS) and a block copolymer of poly(styrene-butadiene) (SBS). Two series of polymer composites with different compositions have been prepared: the first by extrusion followed by injection and the second by dissolution and evaporation of the solvent from the injected samples. The comparison of the FTIR spectra of the polymer composites with those simulated by addition of the spectra of the component polymers allows the detection of differences attributed to the existence of interactions between both polymers that cause partial miscibility between them. The results obtained are corroborated by differential scanning calorimetry.