Coexisting adsorbate phases in high-coverage decaneselenolate and dodecaneselenolate [CH3(CH2)(n)Se, n = 9 and I I I self-assembled monolayers on Au{ 1111 have been characterized by scanning tunneling microscopy and consist of two types: a densely packed distorted hexagonal lattice incommensurate to the underlying gold substrate, as revealed by the observation of a moire pattern, and a commensurate linear missing-row structure. Examination of the nearest neighbor distances in the tightly packed lattice reveal two distinct repeat distances of 4.90 and 5.20 Angstrom, which complements previous surface X-ray data. The linear missing row structure manifests in several variants of the (root3 x root3root3)R30degrees unit cell differentiated by whether the molecules bind at 2- or 3-fold substrate sites. While the number of molecules within this unit cell is typically two, in some cases an additional alkaneselenolate molecule is located at a site one An atom lower than the rest. The structural conclusions are supported by excellent agreement of experimental lattice parameters and those derived from molecular packing models. Comparison of the alkaneselenolate data with analogous structural phases reported for alkanethiolate monolayers on Au{ 1111 shows that differences between the two systems can be understood on the basis that self-assembly is guided both by headgroup-headgroup as well as headgroup-substrate interactions.