Perylenetetracarboxydiimide (PEDI) shows a unique fluorescence property. The dye is highly fluorescent in 3,3＇4,4＇-biphenyltetracarboxylic dianhydride (BPDA)-derived polyimide (PI) matrices but turns nonfluorescent in PIs from pyromellitic dianhydride (PMDA). The results can be explained in terms of a fluorescence quenching mechanism related to the charge transfer ability of the PI matrices. The miscibilities of binary blends between BPDA-derived PI and PMDA-derived PT were studied by applying this fluorescence behavior. In these blends, only BPDA-derived PIs were labeled by copolymerizing a trace amount of bifunctional PEDI. The PI/PI blend miscibilities were found to increase with an increase in drying (casting) temperature for mixed polyamic acid (PAA) solutions, a decrease in the molecular weights of PAAs, and an increase in storage temperature and its period for the mixed PAA solutions. In all cases, the fluorescence intensities decreased in accordance with an increase in the PI/PI blend miscibilities. The results reveal that the PEDE fluorescence is a powerful tool for studying the miscibility of PI/PI blends without appreciable T-g＇s.