The influence of the proportions of the meta- and para-isomers of divinylbenzene and ethylstyrene in the degree of sulfonation of macroporous poly(styrene-co-divinylbenzene) (poly(ST-co-DVB)) beads has been investigated. The first step in this work was the synthesis of poly(ST-co-DVB) beads, with different porous structures, by variation of the proportions of cross-linking agent (%DVB) and of the volume fraction of monomers in the organic phase (Fm) of the polymerization mixture. The next step involved transformation of the polymeric beads obtained into cation exchange resins by means of hot sulphuric acid treatment. The quantitative extent of the sulfonation reaction was evaluated by measuring the ion exchange capacity (q) of each type of resin. The Design of Experiments (DOE) methodology was used to study the effects of %DVB and Fm on q and it was found that only the %DVB factor exerted a significant effect on this parameter, its influence being non-linear and negative. To obtain more reliable results, a new set of experiment was performed in which the range of %DVB values was expanded, covering from 25% to 80% DVB. The results indicated again that the influence of the %DVB factor on q was non-linear and negative; that is, an increase of %DVB in the synthesis process led to a decrease in q. Since this effect can be thought to be due to the hindered penetration of the sulfonation mixture into the very cross-linked gel-type nanoparticles comprising the beads, an attempt was made to find an interrelationship between the specific surface area (S-BET) of each type of bead and their q, trying to determine whether the sulfonation was merely a surface process. However. it was found that not only did sulfonation of the surface of the nanoparticles occur but also that the sulfonation agent penetrated into their interior. Taking these observations into account, it was suspected that one or more of the isomers present in the commercial DVB polymerize in such a way that their sulfonation is not complete. To check this possibility, the proportion of the DVB isomers and q were correlated The quantitative relationship found indicated that the absence of a completely homogeneous sulfonation of poly(ST-co-DVB) beads seems to be due to an insufficient sulfonation of the benzene rings of the para-isomers present in commercial DVB. (C) 2008 Elsevier Ltd. All rights reserved.