Novel solid acids with improved stability were the first time synthesized through biomass carbonization with supercritical ethanol and subsequent sulfonation of alcohothermal chars. Sulfonic groups and other oxygenated functional groups were easier to be loaded on alcohothermal chars than hydrothermal and pyrolytic chars during sulfonation, and exhibited better stability. For the esterification of oleic acid at 80 degrees C, sulfonated alcohothermal chars lost 9.0-18.9% of their activity after four reaction cycles, while it was 13.0-30.8% for sulfonated hydrothermal chars and more than 58.4-71.3% for sulfonated pyrolytic chars. As a result, C(HY, E)-S and C(Glu, E)-S still gave esterification yield of > 83% after four reaction cycles. Sulfonated pyrolytic chars exhibited oxygenated functional structure bonded with mainly C=O linkage, which possibly contributed to their worse performance in esterification than sulfonated solvothermal chars.