Composition profiles generated by interdiffusion in the concentrated regime between poly(phenylene oxide)-polystyrene (PPO-PS) blend pairs are experimentally determined by two techniques. Three-point bending moment measurements over a convenient temperature range (DMA (dynamic mechanical analysis) method) are used to determine interphase composition profiles. Confocal micro-Raman spectroscopy is also used to measure local compositions along a direction which is perpendicular to the original interface. The study includes some limiting cases, to test accuracy, precision and flexibility of the "DMA method". Excellent agreement is found between results obtained by both independent methods.