In situ UV/Vis spectroscopy was used to study supercritical fluid impregnation of azo-dyes with varying functional groups into poly(methyl methacrylate) (PMMA) films. The diffusion and phase behavior of Disperse Orange 25 (DO25) and a dye mixture of Disperse Red I and DO25 in PMMA/CO2 system were studied. Although these azo-dyes are relatively insoluble in supercritical carbon dioxide (scCO(2)) ( < 10(-6) M under conditions of our experiments), the high partition coefficients (approximate to 10(4)-10(5)) between PMMA and scCO(2) phase allow incorporation of similar to 1% of these dyes into PMNIA matrix. The diffusion of azo-dyes in scCO(2)-swollen PMMA film was observed in situ via UV/Vis spectroscopy, and can be tuned simply by changing the CO2 pressure surrounding the polymer film. This article reports the effects Of CO2 pressures and specific intermolecular interactions on the diffusion rate (or diffusivity) and the partitioning of dye solutes between the fluid phase and the PMMA. It also demonstrates a possibility of enhancing diffusion by a dye mixture that is capable of mutual intermolecular interactions and thus reducing interactions with the polymer matrix. (C) 2002 Elsevier Science B.V. All rights reserved.