Full and shortened single-walled and multiple-walled carbon nanotubes were suspended in water to form stable suspensions in the presence of a surfactant. Optical limiting properties of the suspensions were determined for 532-nm pulsed laser irradiation, and the results were comparable with those of carbon black aqueous suspension. Solubilization of the shortened carbon nanotubes was achieved by attaching the nanotubes to highly soluble poly(propionylethylenimine-co-ethylenimine) or by functionalizing the nanotubes with octadecylamine. The soluble carbon nanotube samples formed homogeneous solutions in room-temperature chloroform. Optical limiting properties of these solutions were also determined for 532-nm pulsed laser irradiation, and the results were found to be quite different from those of the carbon nanotube aqueous suspensions. Apparently, the carbon nanotubes exhibit significantly weaker optical limiting responses in homogeneous solutions than in suspensions. Mechanistic implications of the experimental results are discussed.