THE structure and phase behaviour of organic thin films result from the subtle interplay of intermolecular Van der Waals interactions, which promote self-assembly and long-ranged order, and the more complex interactions between the end groups bf the organic chains and the substrate. The structure of molecular films of amphiphiles has been extensively studied on subphases of dielectric liquids, notably water (Langmuir monolayers) and on solid surfaces (self-assembled monolayers, SAMs)(1-4). Here we report structural studies, by synchrotron X-ray re scattering, of an Intermediate case : densely packed alkanethiol films on the surface of liquid mercury, White, like SAMs, these films form strong chemical bonds to the subphase, this subphase is smooth and unstructured, as in the case of Langmuir monolayers, But unlike either of these(1,2,5-7), our films have no inplane long-range order, We suggest that the strong interaction of the thiol group with the underlying disordered liquid dominates here over the order-promoting interactions of the alkyl chains.