The multiwall C nanotubes (CNT) were oxidized with concd. HNO3 or its mixt. with H2SO4 optionally under ultrasonication. Thermogravimetric anal., Raman spectroscopy and Boehm method, as well as detn. of heat of immersion in H2O and zeta potential were used to characterize the CNT surface modification. The large amt. of acidic functional groups (2.23 mmol/g) was obtained when a mixt. of acids and ultrasonication for 6 h were used. The longtime contributed however to a significant decompn. of the C structure.