The basicity constants of a series of para-substituted tetraphenylporphyrins have been determined by two-phase spectrophotometric titration with pH monitoring in the chloroform–aqueous electrolyte solution system; the pH ranges of prevailing of the dication and the neutral forms of the porphyrins have been estimated. With introducing the donor substituents at the para-position of phenyl fragments of porphyrins, the compound basicity increases. Ionization constant of the pyrrole nitrogen atoms depends on the donor-acceptor properties of the substituents according to the Hammett equation. Basing on the derived dependence, theoretical basicity constants of the para-substituted porphyrins bearing strongly donor groups have been predicted.