The adsorption of pentacene (C22H14) on Ag/Si(1 1 1)-(\root3 x root3)R30degrees has been investigated using ultrahigh-vacuum scanning tunneling microscopy at room temperature (RT). Three distinct phases are observed as a function of pentacene coverage. At low coverage the molecules adopt a head-to-head orientation forming a molecular row structure. At a coverage of one monolayer (1 ML), the molecules adsorb between the rows to adopt a close-packed side-by-side orientation, which is mediated by a van der Waals interaction. Higher coverage lead to a molecular reorganization which is interpreted as a tilting of the pentacene molecules from a planar to an upright configuration. (C) 2003 Elsevier Science B.V. All rights reserved.