The transient nonlinear optical response of some recently synthesized symmetric nickel dithiolene complexes, namely the Ni(etodddt)(2) (etodddt = 4,5-(1,4-oxathiane-2,3-diyldithio)-1,3-dithiole), the Ni(pddt)(2) (i.e., bis(6,7-dihydro-5H-[1,4]dithiepine-2,3-ddithiole)nickel)) and the Ni(dddt)(2) (bis(5,6-dihydro-1,4-dithiine-2,3-dithiole) nickel), is studied using Z-scan technique employing 1064 and 532 nm, nanosecond laser pulses. The absorptive and the refractive parts of the third-order susceptibility chi((3)) were determined and the second hyperpolarizability gamma of each dithiolene complex was deduced at both excitation wavelengths, while strong resonance enhancement was found to occur under 1064 nm excitation. The present results are discussed and compared with other literature results obtained on similar symmetric and unsymmetric dithiolene complexes. (C) 2010 Elsevier B.V. All rights reserved.