Single-walled carbon nanotubes (SWNTs) were solubilized by attaching functional groups to the nanotube-bound carboxylic acids in the esterification and various amidation reactions. The solubility made it possible to not only characterize the functionalized SWNT samples in solution but also quantitatively measure the UV/vis/NIR absorption spectra of the samples. The nanotube contents in the soluble samples were determined in terms of NMR signal integrations in reference to internal standards and through thermal gravimetric analyses. The absorptivity results thus obtained are similar for SWNTs in the different functionalized samples. For the near-infrared absorption band corresponding to the first pair of van Hove singularities in the electronic density of states for semiconducting SWNTs, the peak absorptivity is 0.5-2.2 (mg/mL)(-1) cm(-1). The absorption properties of SWNTs are apparently insensitive to changes in the sample environment, such as the functionalization with significantly different groups. The effects of scattering on the accurate determination of absorptivity are discussed.