We present a new determination method of the porosity created by the adsorption space between closed multiwalled carbon nanotubes. Using the grand canonical Monte Carlo (GCMC) simulation of nitrogen adsorption at 77 K and applying the "Karolina" algorithm, local isotherms were simulated for different distances between parallel nanotubes and finally the equation of the global adsorption isotherm was solved. This methodology leads to a satisfactory description of the experimental nitrogen adsorption data showing that the distance between nanotubes is in the range between 4 and 14 nm. (C) 2007 Elsevier Inc. All rights reserved.