a b s t r a c t Degenerate four-wave mixing (DFWM) experiments have been performed to determine the third-order nonlinear optical (NLO) susceptibilities (v (3)) of gold(III) maleimide dithiolate tetraphenylphosphonium, (PPh 4)[Au(midt) 2 ], (Au-P) and gold(III) maleimide dithiolate melamine melaminium hybrid solvate, (C 3-N 6 H 6)(C 3 N 6 H þ 7)[Au(midt) 2 ] À Á2DMFÁ2H 2 O, (Au-Mel). Ab-initio quantum mechanical calculations (time-dependent Hartree–Fock (TDHF) method) of Au-P and Au-Mel have been carried out to compute the elec-tric dipole moment (l), the dispersion-free and frequency-dependent dipole polarizability (a) and the second hyperpolarizability (c) values. These theoretical calculations are in good agreement with the experimentally obtained results by the DFWM technique. All the investigations show clearly the effect played by the counter ion on the resulting NLO properties of the two gold complexes. Ó 2013 Elsevier B.V. All rights reserved.