Basic properties of bis[5-(3-oxyphenyl)-2,8,12,18-tetrabutyl-3,7,13,17-tetramethylporphyrinyl]-3,6,9-trioxyundecane-1,11 and bis[5-(3-oxyphenyl)-2,8,12,18-tetrabutyl-3,7,13,17-tetramethylporphyrinyl]-3,6,9,12-tetraoxytetradecane-1,14 were studied by the spectrophotometric titration in ethanol-sulfuric acid system at 298 K. It was established that protonation of nitrogen atoms in tetrapyrrole fragments of dimeric porphyrins proceeds in two steps. The corresponding ionization constants and the concentration limits of existence of di- and tetracationic forms of the compounds under study were evaluated.