A series of modified polymers with 4-trimethylsilyl-alpha-hydroxylbenzyl (HBTMS) substituents were made as new materials for membrane gas separation. HBTMS was introduced onto polysulfone (PSf), tetramethylpolysulfone (TMPSf), and hexafluoropolysulfone (6FPSf) by forming lithiated polymer intermediates followed by treatment with an electrophile, p-trimethylsilylbenzaldehyde. The resulting side substituent contains the bulky trimethylsilyl (TMS) groups separated from the polymer chain by an aromatic ring. The polymer structures were characterized by NMR spectroscopy, and thermal properties were investigated by differential scanning calorimetry and thermogravimetry. The permeability coefficients of the resulting polymers PSf-HBTMS, TMPSf-HBTMS, and 6FPSf-HBTMS showed considerable increases over the unmodified polymers. TMPSf-HBTMS showed the greatest improvements in gas transport properties with large increases in both CO2 and O-2 permeabilities of similar to3-fold (72 and 18 Barrers, respectively) compared with TMPSf. In the case of the CO2/N-2 gas pair, there was an absence of the typical accompanying tradeoff shown by decreases in permselectivity with increasing permeability. Comparisons of this series of polymers containing the new HBTMS substituent were made with those having other TMS pendant groups.