There have been substantial developments in the instrumentation available at central facilities for neutron and X-ray reflectometry measurements . However, there are still significant differences in the models used in the analysis of reflectometry measurements from insoluble surfactant monolayers. Whilst this problem is not limited to insoluble monolayers, this work will focus on this particular application, however, the methods are easily generalised. The differences in analytical models are covered well in the recent work of Campbell et al. , which, in particular, highlights the inconsistency in the use of interfacial roughness in the analytical models , the variation in the number of layers used to describe the system , and the lack of consideration * andrew.mccluskey@diamond.ac.uk; a.r.mccluskey@bath.ac.uk given to the compaction of acyl chains at high surface pressures .