Floating bilayers are supported phospholipid membranes, which are minimally constrained by the influence of the substrate, and therefore considered to be more realistic than other deposited systems. The technology has been developed over a number of years at ISIS/ILL, and we are now able to make very high quality, essentially defect free membranes, repeatably and with a high degree of control over structure and composition. We have recently carried out a detailed structural analysis of a DPPC FSB, and found that the structural parameters are very close to those of membranes measured by other techniques. However, an analysis of the uncertainty of our fitted values shows that some parameters are very poorly determined. In this proposal, we argue that repeating the measurements with differing levels of deuteration should allow us to reduce these uncertainties.