Poly(benzimidazole) (PSI): poly(acrylic acid) (PAA) blend membranes were prepared with different number-average molecular weights ((M) over bar (n)) and molar ratios of PAA as novel proton exchange membranes. PAA titration behavior showed that by increasing the (M) over bar (n), apparent acidity of PAA decreases. The interaction between PBI and PAA was confirmed by FITR and DSC. The cross-section SEM photographs showed that pore structure can be formed in the membrane by increasing (M) over bar (n) and molar ratio of pAA. The membranes showed lower methanol permeability (1.06-1.91 x 10(-9) cm(2).s(-1)) in comparison to the Nafion and mechanical properties were comparable with phosphoric acid (PA) doped PBI membranes. An increasing trend in proton conductivity of the membranes with (M) over bar (n) and molar ratio of PAA was observed that is attributed to the increasing the water uptake, amount of carboxylic acid groups of PAA and pores in structure where more hydrogen bonding network between PBI and PAA can be formed. Also, by increasing the (M) over bar (n), acidity of PAA was decreased, resulting in more proton transport by hopping mechanism. PBI:PAA100KD(1:4) (membrane with PAA of 100,000 g.mol(-1) and PBI:PAA molar ratio of 1:4) blend membrane showed the highest proton conductivities of 5.0 mS.cm(-1) at 150 degrees C in the anhydrous state; 7.5 mS.cm(-1) at 80 degrees C in the humidified state and 500.0 inS.cm(-1) at 150 degrees C in the H3PO4-doped form.