The effects of adsorbate on the adsorption in a bundle of carbon nanotubes are investigated to explore the preferential adsorption over various adsorption sites: inside the tube, in the cusp interstices and in the square interstices outside the tubes. This is carried Out with the Grand Canonical Monte Carlo simulation and the Simulation results are tested against the experimental results of bundles of single wall carbon nanotubes (SWCN). With regard to adsorbate, we choose argon and nitrogen to represent simple fluids and water to represent strong associating fluids with strong orientation interaction. The preferential adsorption of argon and nitrogen depends on the tube size. For tube size smaller than 10.8 angstrom, adsorption inside the tube is preferred because the solid-fluid potential is greatest in the tube interiors. While for larger tubes adsorption occurs initially in the small Cusp interstices between the tubes, and as adsorption is progressed adsorption occurs inside the tube as well as the larger square interstices. At higher pressures capillary condensation occurs in the square interstices. For water, however, the adsorption mechanism is different. Its adsorption occurs dominantly inside the tube, irrespective of the tube size. This is due to the requirement of appropriate geometry to allow hydrogen bonding among water Molecules to occur. The small cusp interstices do not provide proper space for clusters of hydrogen bonded waters, while the larger square interstices are too large and hence the solid-fluid potential is not strong enough to induce adsorption unless the partial pressure is sufficiently high. Finally the model of these fluids and carbon nanotube is tested with the experimental data of a commercial SWCN, and the Simulation results are in agreement with the data. (C) 2008 Elsevier Inc. All rights reserved.