Polysulfones (PSfs) containing pendant aldehyde groups have potential uses as reactive polymer supports or affinity membranes to bind enzymes and ligands. The polymeric aldehydes may also be utilized to prepare crosslinked membranes and to covalently bond inorganic species to the matrix. A series of polymers containing pendant aldehydes with degrees of substitution (DS) ranging from 0.1 to 2.0 groups per repeat unit was prepared by lithiation at the orthosulfone site and then treatment of the intermediate with dimethylformamide (DMF), a formyl equivalent electrophile. A polymer with aldehyde groups (DS 2) at the orthoether site was also prepared by lithiation of brominated PSf followed by DMF. The new polymer structures were characterized in detail by NMR spectroscopy, and their thermal properties were investigated by DSC and thermogravimetric analysis. (C) 2001 John Wiley & Sons, Inc.