Absorption and electroabsorption measurements were performed on the substituted tetrakis tert-butyl metal-free phthalocyanine (H(2)Pc(t-bu)(4)) solid solution. The complex third-order nonlinear susceptibilities were determined in poly(methyl methacrylate) films doped with H(2)Pc(t-bu)(4). The real and imaginary parts of chi((3))(-omega;omega,0,0) were -5x10(-13) and -4x10(-13) esu, respectively, at 10 wt% doping of H(2)Pc(t-bu)(4). In guest/host system, the aggregation reduced the optical nonlinear susceptibilities rather than optical absorption coefficients.