Oxidized multiwalled carbon nanotubes were produced by strong acid treatment with the aid of sonication. Short-length carbon nanotubes obtained by filtration of the oxidized carbon nanotubes were analyzed by means of X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopies, and scanning electron microscopy. Direct evidence of the modifications of the electronic properties of the carbon nanotubes was obtained by the valence band analysis. The oxidation induced by the acid treatment led to deep changes of the carbon nanotubes density of states. Analysis of the photoluminescence spectra showed that the short carbon nanotubes are luminescent in the visible range. Interestingly, the spectral intensities and lineshapes are strongly dependent on the pH of the CNT solution. This effect is explained by a modification of the local environment of the defects induced by the protonation/deprotonation of the carboxylic groups formed by the oxidation process.