We present changes in the first and second order Raman spectra of multi-walled carbon nanotubes (MWNTs) induced by wet chemical methods. The MWNTs studied were initially treated in HCl to remove the metal growth catalyst and subsequently were subjected to two acid purification routes: 1) reflux in HNO<sub>3</sub> acid and 2) ultrasonification in 3 HNO<sub>3</sub> :1H<sub>2</sub>SO<sub>4</sub>. Raman spectroscopy, using two laser excitation wavelengths (514.5 and 632.8 nm) and XPS were employed to study the evolution of the products. XPS analysis confirmed the presence of oxygen functionalities and electron extraction phenomena. Charge transfer phenomena were observed by a shift of the C1s core level towards higher binding energies. We found that the intensity of both the D and G energy Raman modes if normalized to the second order mode D* mode follow similar trends upon acid treatments. We interpret this result together with the observed dispersion of G mode as an indication that the G mode in carbon nanotubes is defect induced, in a double resonant process. Both acid schemes cause an up-shift of D and G Raman modes, due to intercalation of acid molecules, exerting pressure on the sp<sup>2</sup> structure and an electron transfer from the p states in MWNTs to the oxygen atoms.