This article discusses the diffusion and solubility behavior of methanol/ methyl tert-butyl ether (MTBE) in glassy 6FDA-ODA polyimide prepared from hexafluoroisopropylidene 2,2-bis(phthalic anhydride) (6FDA) and oxydianiline (ODA). The diffusion coefficients and sorption isotherm of methanol vapor in 6FDA-ODA polyimide at various pressures and film thicknesses were obtained with a McBain-type vapor sorption apparatus. Methanol/MTBE mixed-liquid sorption isotherms were obtained by head-space chromatography and compared with a pure methanol sorption isotherm obtained with a quartz spring balance. Methanol sorption isotherms obtained with the two methods were almost identical. Both methanol sorption isotherms obeyed the dual-mode model at a lower activity, which is typical for glassy polymer behavior. The MTBE was readily sorbed into the polymer in the presence of methanol, but the MTBE sorption isotherm exhibited a highly nonideal behavior. The MTBE sorption levels were a strong function of the methanol sorption level. Methanol diffusion in the polymer was analyzed in terms of the partial immobilization model with model parameters obtained from average diffusion coefficients and the dual-mode sorption parameters. Simple average diffusion coefficients were obtained from sorption kinetics experiments, whereas the dual-mode sorption parameters were obtained from equilibrium methanol sorption experiments. An analysis of the mobility and solubility data for methanol indicated that methanol tends to form clusters at higher sorption levels.