Optical limiting properties of a series of mono- and multiple-functionalized fullerene derivatives were investigated systematically in room-temperature solutions at different concentrations. The results provide overwhelming experimental evidence for the strong concentration dependence of optical limiting in fullerene solutions. Possible explanations of the experimental results in terms of nonlinear scattering and nonlinear absorption mechanisms are considered. For the latter, the involvement of excited state bimolecular processes is proposed. Within the context of reverse saturable absorption, the strong concentration dependence of optical limiting in fullerene solutions is explained in terms of the formation of strongly absorptive transient species from the excited state bimolecular processes. The bimolecular processes are probably dependent not only on the solution concentration but also on the molecular diffusivity or medium viscosity, which thus accounts for the fact that the optical limiting responses in a highly dilute solution agrees well with those in a polymer matrix. The relationship of such a mechanism that involves excited state bimolecular processes with the classical five-level reverse saturable absorption model is discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)51209-4].