Adsorption and self-organization of sodium alkyl sulfonates (STS and SHS) on HOPG have been studied by using in situ scanning tunneling microscopy (STM). Both SHS and STS molecules adsorb on HOPG surface and form long-range-well-ordered monolayers. The molecular rows and the axes of alkyl chain of the molecules cross each other at angles of 60degrees and 90degrees in the STS and SHS layers, respectively. Molecular details such as sulfonate functional group (head) and alkyl chain are clearly, imaged. The neighboring molecules in different rows form a "head to head" configuration. Structural models for the molecular arrangement of the two adlayers are proposed. (C) 2004 Elsevier B.V. All rights reserved.