The influence of miscibility on the transport properties of polymer electrolyte blends composed of a proton conductor and an insulator was investigated. The proton-conductive component in the blends was sulfonated poly(ether ketone ketone) (SPEKK), while the nonconductive component was either poly(ether imide) (PEI) or poly(ether sulfone) (PES). The phase behavior of PEI-SPEKK blends was strongly influenced by the sulfonation level of the SPEKK. At low sulfonation levels (ion-exchange capacity (IEC) = 0.8 meq/g), the blends were miscible, while at a slightly higher level (IEC = 1.1 meq/g), they were only partially miscible and for IEC >= 1.4 meq/g they were effectively immiscible over the entire composition range. The PES-SPEKK blends were miscible over the entire range of SPEKK IEC considered in this study (0.8-2.2 meq/g). At high IEC (2.2 meq/g) and at low mass fractions of SPEKK (< 0.5), the miscible blends (PES-SPEKK) had higher proton conductivities and methanol permeabilities than the immiscible ones (PEI-SPEKK). The opposite relationship was observed for high mass fractions of SPEKK (> 0.5). This behavior was explained by the differences in morphology between these two blend systems. At low IEC of SPEKK (0.8 meq/g), where both PEI-SPEKK and PES-SPEKK blend systems exhibited miscibility; the transport properties were not significantly different. (c) 2006 Wiley Periodicals, Inc.