ABSTRACT. The protonation of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin was investigated in aqueous solutions of N,N-dimethyformamide at 25 C and 0.1 mol.dm -3 sodium perchlorate. The solvent effect on value of protonation constant was examined by using the linear solvation energy relationship concept. The value of logK1, logK2 and logKt was correlated with the macroscopic (dielectric constant) and microscopic Kamlet-Taft parameters (,  and *) of binary mixtures. The solvent effects were analyzed in the terms of Kamlet, Abboud and Taft model (KAT). Multiple linear regression were used to find the contribution of the microscopic parameters containing  (hydrogen-bond acidity), * (dipolarity/polarizability) and  (hydrogen-bond basicity). It was found that  and  were the most predominant descriptors. Also, relationship with reciprocal of dielectric constant was obtained based on Born's model, showing the significance of specific solute-solvent interactions. Therefore the hydrogen bonding interactions between solute and solvent components are mainly responsible for the change in protonation constants of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin in water-N,N-dimethyformamid binary mixtures.