The filling of carbon nanotubes (CNTs) with water and acetonitrile in the presence of adsorbed argon at room temperature and atmospheric pressure was investigated using molecular dynamics simulations. The adsorbed gas was found to be an important factor which influences the solvent penetration inside the smallest nanotubes. However, for CNTs with a diameter of more than 1.5 nm, including CNT(11, 11), the role of gas is negligible although it cannot be spontaneously removed. At 298 K acetonitrile fills pristine carbon nanotubes more than 10 times faster than water due to its higher mobility and less surface tension in spite of a larger polarity of the molecule. (C) 2010 Elsevier B.V. All rights reserved.