We report a diffusion study oil a series of interphases formed between a polystyrene-rich (PS) liquid layer and a poly(phenylene oxide) (PPO) glassy matrix. Diffusion was promoted by annealing the polymer pair tit several temperatures below the glass transition temperature of the PPO matrix, in experiments where the liquid component was supplied from an unlimited source. Depth PS concentration profiles were obtained via optical sectioning through the glassy layer with confocal Raman microspectroscopy. The PS profiles measured ill all the samples had sharp diffusion fronts followed by a region with relatively uniform PS concentration. From the time evolution of the PS front advance, we directly obtained the time-scaling laws for interphase kinetics which are interpreted in the context of the Fickean and case II diffusion theories. Despite some recent Studies reporting the occurrence of case II in this particular system, our results and analysis show conclusively that interphase kinetics are, oil the contrary, markedly Fickean. Results previously published by other authors on this polymer pair are also analyzed with the aim of offering a unified view about the mechanism that controls interphase evolution tit the molecular level.