For the analysis of ultrathin layer systems with partial order, we demonstrate a navel Patterson-function analysis of the small-angle X-ray scattering which does not rely on any detailed or unphysical assumptions about film structure and order. The one-dimensional overlayer Patterson function of the film is shown to present the information from the measurements in real space in an intuitively accessible form and to be more appropriate for partially ordered systems than the box models in common use. We apply the analysis to determine the structure of multilayer Langmuir-Blodgett films of a number of sidechain-substituted polysiloxane derivatives. We demonstrate that it allows conclusions to be drawn about the asymptotic decay of layer order correlations and the detailed distribution of electron density within the bilayer repeat unit, both of which are inaccessible to box-model methods.