The functionalization by nitric acid vapors at azeotropic concentration has been recently proposed to eliminate drawbacks of the widely utilized liquid phase functionalization method. This work suggests to exploit the so-called "salt effect" to improve the vapor phase oxidation method in terms of safety and flexibility. Increasing the relative volatility of acid, the addition of Mg(NO3)(2) salt to the HNO3 + H2O solution allows (i) obtaining vapors with HNO3 at the azeotropic concentration from a more diluted liquid solution ( i.e. operating under safer conditions), and (ii) varying the concentration of HNO3 in the vapor phase even above the azeotropic concentration limit ( with improved process flexibility). High-resolution transmission electron microscopy, thermo-gravimetry, Raman spectroscopy and X-ray photoemission spectroscopy systematic analyses are carried out on pristine and oxidized nanotubes in order to assess their functionalization degree, surface chemistry and structural evolution. The most relevant finding of this preliminary study is that the nanotube functionalization extent increases linearly with the HNO3 vapor concentration. (C) 2014 Elsevier B. V. All rights reserved.