There is a continuing interest in determining the detonation characteristics of loaded plastic-bonded explosives (PBXs). Within the UK, the relevant government agencies wish to better understand the response of insensitive high explosives. This has required more detailed investigation of the transit of reaction from the unreacted state to products. The starting condition for shock to detonation transition (SDT), before application of a kinetic scheme to describe reaction, is thus the unreacted Hugoniot for the material. In this work, three PBXs, manufactured by BAE Land Systems, are investigated and modelled. All contain RDX in differing quantities in an HTPB binder. One of them contains aluminium. Two of the materials have the same weight percentage of filler and binder but differ in the grain size distribution entrained. The experimental Hugoniots are presented, and a composite equation of state is derived using an engineering model, and a model based on continuum mixture theory. Both are shown to describe the measurements well. Further applications of the technique are described and future uses are outlined.