In this work, Ca(OH)(2) nanoplates supported on activated carbon (AC) were used to efficiently remove free fatty acids (FFA) from soybean oil. SEM, TEM, XRD, TG, Zeta potential measurements showed that Ca(OH)(2) nanoplates with average dimensions of 100-300 nm and crystallite size 17 nm supported/dispersed on the surface of the AC are very efficient to remove FFA by a reactive adsorption. Experiments with oleic acid 20% in soybean oil and Ca(OH)(2) dosages varying from 5 to 60 mg mL(-1) reached near 100% removal of FFA. The data suggest a reactive adsorption of oleic acid (OA) to form Ca(oleate)(2) impregnated on the AC surface which can be easily removed from soybean oil. On the other hand, pure Ca(OH)(2) nanoparticles or commercial reacts with OA to form a paste which cannot be separated. The Ca(OH)(2)/AC composite can be used as a FFA trap to allow biodiesel synthesis from soybean oil with 5 and 10% FFA using the classical basic NaOH homogeneous catalyst.