A new single-step method of multiwalled carbon nanotubes opening is reported. It bases on hydrothermal treatment with H2O2 solution, so in this way one reduces energy and water consuming purification process. The efficiency of a new method is compared for the same tubes but opened by the 'typical' procedure using treatment with molten alkaline mixture. The data of low temperature N-2 adsorption and TEM measurements indicate that under specific conditions one can easily open the tubes. Enthalpy of immersion in water measurements are reported showing that application of this new procedure might lead to simultaneous creation of surface oxygen-containing functional groups. (C) 2009 Elsevier B. V. All rights reserved.