The basicity of the meso-porphyrins, namely meso-tetraphenylporphyrin (I), meso-tetrakis(4-methylphenyl)porphyrin (II), meso-tetrakis@-methoxyphenyBporphyrin (III), meso-tetrakis(4-aminophenyl)porphyrin (IV), meso-tetrakis(4-chlorophenyl)porphyrin 0 and meso-tetrakis(4-nitrophenyl)porphyrin (VI), was investigated potentiometritally in nitrobenzene solvent. This investigation showed that these compounds are basic, rather than acidic. Whereas they cannot be titrated even with tetrabutylammonium hydroxide, they can readily be titrated with perchloric acid to give well shaped and stoichiometric end-points. In addition, they undergo two proton reactions per porphyrin molecule..However, IV shows a second end-point corresponding to a six-proton reaction per porphyrin molecule. Half-neutralization potentials (measures of their basic&y) of these compounds are I = 321, II = 270, III = 255, IV = 155, V = 395 and VI = 480 mV. These values clearly indicate that, if para-hydrogen with respect to the bond found to be between the phenyl group and porphyrin core of meso-tetraphenylporphyrin (I) is replaced with basifying methyl, methoxy and amino groups, the basicity of I increases (270, 255 and 155 mV, respectively); if the same hydrogen atom is replaced with acidifying chlorine and nitro groups, the basicity of I decreases (395 and 480 mV, respectively). These observations show that the nitrogen atoms at the centre of the prophyrin molecules are strongly influenced by changes even at the perifery of the molecules, which is a good indication that porphyrin molecules are flat.