The removal of free fatty acids (FFA) in vegetable oils is an important pretreatment in the production of biodiesel, in particular, when the starting materials are low-cost feedstocks. Heterogeneously catalyzed esterification with methanol transforms FFA in fatty acid methyl ester (FAME), decreasing the oil acidity and producing biodiesel simultaneously. The equilibrium of this reaction shifts toward the desired product when increasing the methanol content, but at the same time, a double-liquid-phase system forms when the methanol content is higher than 6-8 wt %. The presence of a double liquid phase can be an important drawback in the reactor. A detailed study about the optimization of the methanol quantity is presented, both using a batch and a packed-bed reactor (PBR) at different temperatures (between 60 and 105 degrees C) using Amberlyst 46 (ion-exchange resin) as the heterogeneous catalyst. The deacidification of sunflower oil in a monophasic liquid system leads to satisfactory results (final FFA lower than 0.5 wt %) for both of the reactors. The experimental results demonstrate that the excess of methanol is not convenient in terms of both slower reaction rates and mass of reactant used. The stability of Amberlyst 46 in the PBR was positively verified after 600 h of work.