Sulfonated porous carbons (PCs-SO3H) are prepared by the sol-gel reaction of resorcinol and formaldehyde. The porosity is maintained during drying using a cationic polyelectrolyte as pore stabilizer. It was found that varying the Resorcinol/Na2CO3 molar rate, different resins are produced which, after pyrolysis, give carbonaceous materials with different textural properties. It seems that a Resorcinol/Na2CO3 molar ratio of 200:1 was the optimal condition to produce a well-developed porous structure. Both resins and carbon materials are sulfonated by treatment with sulphuric acid. The relation between the carbon particles size, the sulfonation efficiency and its performance as a catalyst is studied. The higher amount of sulfonic groups, and thermal stability, as well as better catalytic performance, was obtained when smaller porous carbon particles were chosen by sieving. PCs-SO3H exhibited high efficiency for the esterffication reaction and high performance for biodiesel production. The catalysts can be recycled several times with a minimal loss of activity. Thermal analysis evidenced stability up to ca. 200 degrees C, allowing the use of this catalyst at high temperature. The simple synthesis and low cost of the PCs-SO(3)Hs make them promising catalysts for the synthesis of biodiesel. (C) 2016 Elsevier B.V. All rights reserved.