Optical limiting properties of tert-butyl methano[60]fullerene carboxylate were investigated systematically in room-temperature solution at a series of concentrations while the linear transmittance of the solution at 532 nm was kept constant. The results are compared with those of [60]fullerene (C-60) Obtained under the same experimental conditions. For both C-60 and the methano-C-60 derivative, optical limiting responses toward the second harmonic of a Q-switched Nd:YAG nanosecond pulsed laser at 532 nm are strongly dependent on the fullerene solution concentrations. The concentration dependence is not related to any special optical effects because the results of chloroaluminum phthalocyanine as a reference in the optical limiting experiments show no such dependence. Instead, the strong concentration dependence in the optical limiting performance of fullerenes in solution is likely due to concentration effects on optical limiting contributions that are associated with bimolecular excited-state processes in the fullerenes. For an examination of the medium viscosity dependence of the bimolecular excited-state processes, optical limiting responses of the methano-C-60 derivative in highly viscous nonreactive solvent-polymer blends were determined and compared with those in solution at the same linear transmittances. The optical limiting responses are significantly weaker in the highly viscous media, consistent with medium viscosity effects on diffusional or pseudodiffusional bimolecular excited-state processes. Also consistent with such effects are the results that optical limiting responses of the methano-C-60 derivative in poly(methyl methacrylate) polymer films are much weaker than those in room-temperature solution. A reverse saturable absorption mechanism that includes both unimolecular and bimolecular (self-quenching and triplet-triplet annihilation) excited-state processes of fullerenes is proposed. A consistent understanding of the optical limiting properties of fullerenes in room-temperature solution (including the strong concentration dependence), in a highly viscous solvent-polymer blend, and in polymer film is discussed within a single mechanistic framework.