Several types of metallophthalocyanines have been synthesized to establish the relationship between molecular structure and third-order optical non-linearity. Third-order non-linear optical susceptibilities chi((3)) of phthalocyanine (Pc) containing central metal atoms oxovanadium, copper, nickel, iron and cobalt, and peripheral substitutes were measured by the third-harmonic generation technique. The chi((3))(-3 omega; omega, omega, omega) values of VOPc(NH2)(4), CuPc(NH2)(4), NiPc(NH2)(4) and FePc(COOH)(4) were found to be 6.3 x 10(-12), 2.0 x 10(-2), 1.63 x 10(-12) and 1.18 x 10-(12) e.s.u. respectively at 2.1 mu m. Although the donor and acceptor substituents have a dilution effect, chi((3)) was still larger than that of the corresponding unsubstituted phthalocyanines, with the exception of metallophthalocyanines with an axial group where the peripheral substituents required for processability or otherwise are not beneficial for optical non-linearity. In addition, the magnitude of chi((3))(-3 omega; omega, omega, omega) was found to be influenced by different functional groups substituted in cobalt phthalocyanine. The effect of various peripheral substituents on the magnitude of the third-order non-linear optical susceptibility is discussed.