Six-membered ring sulfonated polyimides (SPIs) are promising membrane materials for low temperature (< 100 degrees C) fuel cell application. This paper describes our recent work on the synthesis of various sulfonated diamine monomers and the related sulfonated polyimides. The relationship between the chemical structure and the proton conductivity and water stability of various sulfonated polyimide, membranes is discussed in detail. (c) 2006 Elsevier B.V. All rights reserved.