We investigate the nonlinear optical properties of Cu9S5 nanocrystals using the open aperture Z-scan method with 20 ps and 4 ns laser pulses at 532 nm. We found that optical nonlinearity arises from free carrier absorption in the former case, whereas in the latter it occurs mostly due to nonlinear scattering. Parameters characterizing free carrier absorption and nonlinear scattering were determined. The dynamics of the nonlinear processes was confirmed by pump-probe measurements with 20 ps pulses. Observation of optical limiting at both excitation time regimes indicates the potential of Cu9S5 nanocrystals in laser safety devices. (C) 2014 Elsevier B.V. All rights reserved.