The coverage dependent phase behavior of molecular films of n-alkanes (CH3CHn-2CH3, denote Cn) on mercury was studied for lengths 10less than or equal tonless than or equal to50, using surface tensiometry and surface x-ray diffraction methods. In contrast with Langmuir films on water, where roughly surface-normal molecular orientation is invariably found, alkanes on mercury are always oriented surface-parallel, and show no long-range in-plane order at any surface pressure. At a low coverage a two-dimensional gas phase is found, followed, upon increasing the coverage, by a single condensed layer (nless than or equal to18), a sequence of single and double layers (19less than or equal tonless than or equal to20; ngreater than or equal to26), or a sequence of single, double, and triple layers (22less than or equal tonless than or equal to24). The thermodynamical and structural properties of these layers, as determined from the measurements, are discussed. (C) 2003 American Institute of Physics.