Carbonization of d-glucose at 573−723 K followed by sulfonation produces a functionalized amorphous carbon material with acid catalytic activity as a solid-acid replacement for sulfuric acid. The carbon material contains phenolic hydroxyl, carboxylic acid, and sulfonic acid groups and exhibits high catalytic performance for liquid-phase acid-catalyzed reactions. Carbonization at higher temperature followed by sulfonation also results in amorphous carbon, but the resultant does not exhibit catalytic activity although the amorphous carbon has sufficient amount of sulfonic acid groups. Structural and active site analyses suggest that the marked difference in catalytic activity is due to the accessibility of reactants to sulfonic acid groups in the carbon structure.