Acids solids sulfonated carbons Starbons-300 were prepared by using a mixture of ClSO3H/H2SO4 and H2SO4 (98%) as sulfonating agents. Depending on the sulfonation method used important effects were found on the acidity, textural properties and activity. SO3H groups linked to the Starbons-300 surface were observed by FTIR photoacustic spectroscopy. The functional groups were characterized by NMR C-13 solids and liquids and HSQC liquid of Starbons-300A(2)-H(2)SO(4)AClSO(3)-TEPO. The acidity strength determined with zeta potential measurements and P-31 MAS NMR showed strong acidity in the sulfonated samples. Total number of acid sites was determined by acid-base back titration. The content of SO3H groups was estimated by measuring the cationic exchange capacity. The elemental analysis was carried out by XPS and SEM. The sulfonated Starbons-300 showed very high activity for the esterification of free fatty acids with ethanol. Conversions up to 60% of oleic acid and selectivity to the ester of 100% were reached after 3 h of reaction at 80 degrees C. The results showed that the sulfonated carbon Starbons-300 catalysts are promising acids catalysts to be used in the esterification of free fatty acids with ethanol. (C) 2012 Elsevier Ltd. All rights reserved.