Sulfonated poly(sulfide sulfone imide) copolymers containing flexible sulfide bond and six-membered imide ring were synthesized by random polycondensation. Two types of membranes were prepared by using sulfide (S-PSI) and sulfide sulfone (S-PSFI) based non-sulfonated diamines to investigate the effects of the hydrophobic component. IECw values were controlled to 1.51-1.94 meq g-1 depending on the degree of sulfonation (DS) which was in the range of 50-80%. The membrane series showed good thermal stability up to 310 °C and mechanical properties (tensile strength >30 MPa). Dimensional stabilities were excellent with 23-35% increases, even at 100 °C. Proton conductivities of membranes composed of different hydrophobic diamines display a relatively good correlation with water content and morphology. In fuel cell tests, the S-PSI60 membrane shows relatively high current density of 250 mA cm-2 at 0.6 V and maximum power density of 175 mW cm-2 at 120 °C, 35% RH, 1.5 atm. ; peer reviewed: yes ; system details: This record was machine loaded using metadata from Scopus ; NRC Pub: yes ; submission instructions: article in press