Diffusion between a liquid polystyrene and a glassy poly(phenylene oxide) matrix is experimentally studied over a wide range of temperatures and diffusion times, using confocal Raman microspectroscopy. A specially designed experimental setup allows precise direct following of time evolution of the chemical composition profiles along the diffusion path. A direct and precise quantification is made for the experimental errors involved in two methods used for Raman measurements. An already proposed diffusion model is used to predict the time evolution of the advancing composition profiles along the diffusion path, and gives precise results. Experimental thermodynamic and kinetic data taken from literature are used for the model calculations, and excellent agreement with experimental results is obtained. Diffusion slow down is confirmed at the lowest diffusion temperature used, and probable causes are discussed.