Abstract: The protonation constants (K₃ and K₄) of 15 porphyrins were determined by Aronoff method, being in the range of 4.3-6.61gKvalues.When the difference between 1gK₃ and 1gK₄ is larger than l,no double protonated cation (PH₄²⁺) is formed before the simple protonated cation(PH₃⁺) has been completely formed; otherwise the formation processes of PH₃⁺ and PH₄²⁺ mix up to some extent.Strong electron with drawing meso-nitro groups make protonation constants decrease through inductive effect while meso-chlorine atoms increase the constants due to the steric effect.