Three commercially available poly-4-vinylpyridine resins, two of gel-type (2% cross-linked) and one macroreticular in nature (25% cross-linked) are investigated in the solid state with SEM and in the swollen state with a combination of inverse steric exclusion chromatography (ISEC) and ESR (test of rotational mobility of the spin probe TEMPONE) in various liquid media. In all cases the materials result to be scarcely swellable but, after iodomethylation they develope a quite noticeable nanoporosity in methanol, water and water/methanol/acetic acid. These observations are particularly valuable in the case of the resin Reilex 425 (Reilly Industries, Inc., USA) that is, in its iodomethylated form, the support of [Rh(CO)(2)I-2]-, i.e. the actual "molecular" catalyst in the innovative Chiyoda-UOP industrial synthesis of acetic acid (Acetica process). (c) 2006 Elsevier B.V. All rights reserved.