The emerging applications of functionalized carbon nanotubes (CNTs) in various research domains necessitate the use of many different analytical techniques to confirm their structural modifications in a fast and reliable manner. Thus far, NMR spectroscopy has not been among the main tools for characterization of organically modified carbon nanostructures. 1H analysis is limited because the signals in these derivatives are typically weak and broad, resulting in uncertainties of a few parts per million, and because of the strong interference of residual solvent signals. To overcome these limitations, we investigated the applicability of proton NMR spectroscopy based on gradient-edited diffusion pulse sequences (1D diffusion-ordered spectroscopy, DOSY) in the characterization of CNT derivatives. In general, diffusion NMR experiments allow the separation of NMR signals of different species present in a mixture, according to their own diffusion coefficients, merging spectroscopy information with size analysis. In the present study, a selected set of CNT derivatives was synthesized and analyzed using 1D DOSY experiments by applying strong magnetic field gradients (up to 42.6 G cm−1). Colorimetric tests (i.e., Kaiser test) and TGA analysis support the NMR findings, which are related to isolated and/or bundled short SWNTs, on the basis of TEM and AFM characterization. The overall results show that the diffusion-based NMR spectroscopy is a fast and promising approach for the characterization of covalently modified CNT derivatives.