An experimental method to determine interphase composition profiles in amorphous polymers pairs and polymer-solvent pairs is presented. The method is based on the measurement of dynamic mechanical properties of slender composite beams, and well-established properties of amorphous polymer homogeneous blends and solutions. The method does not require tracers. A simple calibration procedure is included in the description, and some results for a polystyrene-polystyrene pair are used to illustrate the method application.