The optical limiting characteristics of pure and boron-doped multi-walled carbon nanotubes (MWNTs) are studied and compared at wavelengths of 532 and 1064 nm. Fluence-dependent transmission and Z-scan measurements using 8 ns pulses show that the boron-doped MWNT suspensions exhibit a stronger optical nonlinearity with fluence than pure carbon nanotube suspensions in both the visible and near-infrared wavelength ranges. The nonlinear response for both materials is dependent on reaggregation within the suspension under study. (C) 2004 Elsevier B.V. All rights reserved.