A thermosetting phenolic resin with a pendant sulfonic acid group was prepared by reacting a resol-type phenolic resin (PF) with a Novalak-type sulfonated phenolic resin (SPF). Large amounts of gaseous molecules with similar and small size such as H2O and SO2 evolved in the range of 110 and 350 degreesC during the pyrolysis of this thermosetting phenolic resin (PF/SPF). Highly permeable carbon molecular sieve (CMS) membranes were obtained by pyrolysis of PF/SPF(45/55) precursor membranes which were dip-coated on porous alumina tubes. For example, the membrane pyrolyzed at 500 degreesC for 1.5 h displayed H-2, CO2, and O-2 permeances of 1950, 800, and 240 [GPU (gas permeation units) = 10(-6) cm(8)(STP).s(-1).cm(-2).cmHg(-1)], respectively, and ideal H-2/CH4, CO2/CH4, and O-2/N-2 separation factors of 65, 27, and 5.2 at 35 degreesC and 1 atm, respectively. Sulfonic acid groups linked to thermostable polymer chains might act as "bonded templates" and showed attractive potential in the preparation of CMS membranes.