It was established by the methods of PMR and mass spectrometry that zinc complexes of meso-butyloxymethyletio- and -octaethylporphyrin, when heated in conc. H2SO4, are converted to products of the cationic addition of butylene to carbonium ions of the porphyrins. A possible scheme of formation of the new porphyrins and the significance of the reaction discovered for the chemistry of meso-substituted porphyrins are discussed.