Abstract: The absorption spectra of a series of metal phthalocyanine have been measured and their spectroscopic parameters calculated. They show similar monomer absorption spectra in the visible region with absorption maxima between 650-700 nm. The values of which show a little red shift with increasing solvent polarity. The central metal has little effect on the position of absorption maxima with decreasing order Mg ≥ Zn > Co. Some substituted phthalocyanines form dimers in less polar solvents, since they are non-photoactive, they can not be used as sensitizers. In this paper, the correlations of the formation of dimers with solvent polarity, concentration, nature of substituent group and central metal were investigated. The results show that the formation of dimers depends on the nature of substituent group, dielectric constant of solvent, and concentration. The monomer-dimer equilibrium constants K_eq = C_D/C_M of zinc sulfoamidophthalocyanines in chloroform and ethanol have also been calculated as 1 × 10^-6l. mol^-1.