In this paper we demonstrate that the sonication-driven exfoliation of aggregates and bundles of single-wall carbon nanotubes (SWNTs) in an aqueous surfactant solution can be easily monitored by UV-vis spectroscopy. The different stages of the exfoliation process were directly visualized by cryogenic temperature transmission electron microscopy, showing an excellent correspondence with the spectroscopic data: the maximum achievable exfoliation (which does not mean that 100% of the NTs are effectively exfoliated) corresponds to the maximum UV-vis absorbance of the NT solution. Moreover, it has been observed that NTs produced by the arc-discharge technology (Carbolex NTs) require less energy to achieve maximum exfoliation than NTs produced by chemical vapor deposition (HiPCO NTs). This difference is attributed to weaker van der Waals attraction between Carbolex NTs in the bundles and aggregates.