The third-order nonlinear optical response of fullerene C-84 (isomeric mixture as obtained from fullerenic soot) and its most abundant isomers C-84-D-2 (IV) and C-84-D-2d (II) dissolved in toluene were investigated by the Optical-Kerr-effect technique using 100 fs laser pulses at 830 nm. The second-order hyperpolarizabilities of C-84, D-2 and D-2d were determined to be (0.93 +/- 0.22) x 10(-30), (0.94 +/- 0.22) x 10-(30) and (6.4 +/- 1.2) x 10(-30) esu, respectively. The enhancement of the nonlinearity of the D-2d isomer as compared with the D-2d one is discussed. (c) 2006 Elsevier B.V. All rights reserved.