We show that covalently attached functional groups can alter the densities of individual single-walled carbon nanotubes (SWNTs) in a predictable and highly controllable manner. A volume-additivity model based on molecular group contributions can be used to estimate the density difference between 4-hydroxyphenyl-functionalized and nonfunctionalized HiPco SWNTs as approximately 98.3 kg/m3, compared with 97.9 kg/m3 measured by density-gradient centrifugation. Conversely, the estimated density difference between the (6,5) (0.75 nm diameter) and (9,8) (1.17 nm diameter) SWNTs is smaller at 23.4 kg/m3. We conclude that covalent functionalization can provide an effective handle to separate particular SWNTs from a typical diameter distribution. We show that SWNT mixtures in which metallic SWNTs have been selectively reacted produce two distinct density fractions corresponding to functionalized metallic and pure semiconducting SWNTs. The results were confirmed by Raman spectroscopy, where the high-density fractions exhibit an increased disorder mode with a corresponding decrease in intensity for the low-density fraction. This method also allows for the first independent measure of (n,m) SWNTs having different chemical conversions with functional groups, which will allow for a more rigorous analysis of SWNT chemistry than is possible with uncalibrated spectroscopies such as Raman or photoluminescence.