Adsorption isotherms obtained by Smit et at. using Monte-Carlo simulation for alkane molecule adsorption in the narrow-bore nanotubular pores of Aluminophosphates show a highly unusual structure with chain length dependent steps and whose interpretation is uncertain. An exact calculation of the statistical mechanics of a lattice model of alkane molecule adsorption shows that the isotherm structure arises from the energetic cost of packing molecules into pores and is not directly related to capillary condensation. Three-dimensional interactions are not required to produce the isotherm steps. (C) 2004 Elsevier B.V. All rights reserved.