This paper investigates the third-order nonlinear optical properties of 4 alpha-(4-tert-butylphenoxy) phthalocyaninato dichlorotin(IV) (alpha-SnOtBpPc) and 4 beta-(4-tert-butylphenoxy) phthalocyaninato dichlorotin(IV) (beta-SnOtBpPc) in different organic solvents. The third-order susceptibilities of alpha-SnOtBpPc and beta-SnOtBpPc are reported in different solvents, using Z-scan techniques with 10 ns laser pulses at 532 nm. Their nonlinear absorption coefficient and absorption cross sections were also determined. The molecular imaginary components of the second-order hyperpolarizability Im[gamma] of alpha-SnOtBpPc and beta-SnOtBpPc were found to be 2.60 x 10(-31) and 2.94 x 10(-31) esu (tetrahydrofuran), 2.12 x 10(-31) and 2.54 x 10(-31) esu (chloroform), 3.06 x 10(-31) and 2.54 x 10(-31) esu (dichloromethane), and 1.27 x 10(-31) and 1.50 x 10(-31) esu (toluene), respectively. This study found that substitution at the alpha position has an effect of lowering the two-photon (2PA) cross section value for alpha-SnOtBpPc compared to that for beta-SnOtBpPc, with values of 64.30 and 456.65 GM, respectively. The large 2PA cross section observed in beta-SnOtBpPc is attributed to the decreased energy difference between the virtual state and the LUMO.