Biodiesel can be easily obtained starting from high-quality or refined vegetable oils and performing on these feedstocks a transesterification reaction usually promoted by alkaline catalysts in the homogeneous phase. In this production scheme the employment of low-quality or waste raw materials, as cheap as possible, must be carefully considered to strongly improve the economic competitiveness of this biofuel with respect to the petroleum-derived diesel. In the case of raw materials characterized by a high content of free fatty acids (FFAs), a preliminary esterification treatment with methanol or ethanol is necessary for the abatement of the FFA concentration below the threshold limit value of 0.5-1.0% by weight that is tolerable by the subsequent process step of alkaline transesterification. In the present work the esterification reaction in tubular packed bed reactors, operating under pressure and using a sulfonic acid resin as catalyst, has been extensively studied. As it is well-known, sulfonic exchange resins are subjected to an impressive swelling phenomenon, and it is difficult to obtain, in this case, a dimensionally stable packed bed reactor. A particular solution to this problem has been proposed by the authors using springs of suitable size and shape as catalyst diluent. The influence of operative conditions like overall feed flow rate, reactants feed molar ratio, reactor aspect ratio L/D-R, and mixture viscosity in the described reactors has been investigated. The collected experimental data have been interpreted by means of a mono-dimensional packed bed reactor model in which the external mass-transfer limitation (fluid-to-particle) has been accounted for.