Physisorption of linear-chain alkane and fatty acid molecules on the outer surface of single-wall carbon nanotubes (SWNTs) has been studied by scanning tunneling microscopy (STM) measurements. These molecules are known to organize and form a self-assembled monolayer on a basal plane of graphite, showing a lamellar pattern. We have found that stearic acid molecules (C17H35COOH) also show a lamellar pattern on the SWNT surface. This suggests that stearic acid forms a self-assembled monolayer and modulates the surface charge density of SWNT. Formation of the self-assembled monolayer has been observed on the SWNTs surface less frequently than on the graphite surface. This can be explained in terms of an inhomogeneous chirality distribution of the SWNTs and weak van der Waals interaction between the molecules and SWNTs. (C) 2003 Elsevier B.V. All rights reserved.