Electron and proton conductive properties of Nafion/poly(vinylidene fluroride)-co- trifluoroethylene (PVDF-TrFE) blends were investigated in relation to domain morphology guided by phase diagram using differential scanning calorimetry, polarized optical microscopy, and AC impedance analyzers. A theoretical phase diagram was established by self-consistently solving the combined free energy density of FloryeHuggins theory for liquid-liquid demixing and the phase field theory for crystal solidification. Nafion/PVDF-TrFE blends revealed an hourglass type phase diagram, consisted of single phase crystal (Cr1), liquid þ liquid (L1 þ L2) and crystal þ liquid (Cr1 þ L2) coexistence regions. Guided by the phase diagram, the co-continuous or dispersed droplet domains were produced via phase separation induced either by solvent evaporation or thermal quenching. Fourier transformed infrared spectroscopy and water uptake measurements revealed swelling reduction in the Nafion/PVDF-TrFE blends. Accompanying the ferroelectric to paraelectric transition, the PVDF-TrFE copolymer exhibited a change of capacitor to insulator behavior with increasing temperature. Neat Nafion is poor electron conductor, but it becomes an ion conductor when hydrated. Electron/ion and proton conductivities of the 60/40 Nafion/PVDF-TrFE blend were discussed in relation to the comingled percolated morphology of the membrane.