We measured the yield of positronium (Ps) in sulfonated aromatic proton conducting membranes for the polymer electrolyte fuel cell (PEFC) with and without -SO2- in their chemical structures by positron annihilation lifetime spectroscopy (PALS). It was observed that Ps formation is almost totally inhibited in the polymers without -SO2- Such as sulfonated poly(ether ether ketone) (SPEEK). On the other hand Ps favorably forms in those with -SO2- as sulfonated polyether sulfone (SPES), which is due to the anti-inhibition effect of -SO2-. The high probability of Ps formation in these polymers enables the study of the free volume and the mechanism of gas permeation by PALS.