Experimental results confirm the general rules for miscible polyimide pairs containing the hexafluoroisopropylidene group if (1) the dianhydride composition is the same in each pair and (2) the diamine is changed from the 3,3’ (meta-substituted) 6F-diamine to the 4,4’ (para-substituted) GF-diamine. These rules are also applicable to end-capped polyimide pairs. This conclusion is supported with the calculations of (chi) values based on the Flory-Huggins theory. We also discover two interesting polyimide alloys : 6FDA-PMDA-3,3’-6F-diamine/6FDA-PMDA-4,4/-6F-d forms fully miscible blends, while 6FDA-3,3’-6F-diamine/BTDA-4,4’-6F-diamine forms partially miscible blends (6FDA = hexafluorodianhydride, PMDA = pyromellitic dianhydride, BTDA = benzophenonetetracarboxylic dianhydride). These surprising results confirm that the rigidity, bond angle, space and the length of the dianhydride can significantly affect the molecular packing and miscibility tendency of polyimide blends.