We report on the dependence Of the properties oil the morphologies of sulfonated polymers 3a and 3b (eta(inh) = 0.65-1.18) with segments containing clusters of 6 and 12 sulfonic acid groups, respectively. The larger sulfonated clusters in polymers 3b resulted in higher proton conductivity and much better cell performance than polymers 3a. The highest power density of I fuel cell using 3a-1 (IEC = 1. 16 mequiv/g) and Nafion 117 was 0.23 and 0.21 W/cm(2), respectively, at the effective work potential of 0.5 V, whereas that of 3b-1 (IEC = 1.16 mequiv/g) was 0.29 W/cm(2) at even higher effective work potential of 0.6 V. The morphological structure of 3a-1 and 3b-1 was investigated by transmission electron microscopy (TEM) and compared with that of Nafion, TEM images of 3a-1 and 3b-1 revealed a phase separation similar to that of Nation, which may explain their higher proton conductivities compared to those of randomly sulfonated copolymers.