A series of sulfonated poly(siloxane imide)s (SPSIs) were synthesized from 4,4'-ketone dinaphthalene 1,1',8,8'-tetracarboxylic dianhydride (KDNTDA), a dimethyl siloxane oligomer-based diamine, and a sulfonated diamine. The reduced viscosities ranged from 1.0 to 3.5 dL/g at 35 degrees C in the triethylamine (TEA) salt form. The SPSIs showed anisotropic membrane swelling with larger swelling in thickness than in plane. They displayed reasonably high proton conductivity, thermal stability and good mechanical properties. The KDNTDA-based SPSIs showed good solubility in common aprotic solvents not only in TEA salt form but also in proton form. The H-1-NMR results indicated that the molar content of the dimethyl siloxane oligomer in the SPSIs was 50-80% of that in the feed. (C) 2009 Wiley Periodicals, Lnc. J Appl Polym Sci 112: 3560-3568, 2009