Optical nonlinearities of gold nanoparticles protected by ligands have been investigated by Z-scan technique. Absorptive and refractive optical limiting (OL) effects have been measured with 8 ns duration laser pulses at 532 nm. Significant differences have been found experimentally for these two nanoparticles that differ only by their ligands. Such differences indicate that nonlinear optical responses in two nanocomposites are strongly influenced by the surface plasmon resonance of gold nanoparticles and also the nature of ligands. Both of these two nanocomposites show stronger refractive optical limiting effects than C-60 in toluene. (C) 2002 Published by Elsevier Science B.V.