This paper describes dropweight experiments performed on conventional and ultrafine cyclotrimethylene trinitramine (RDX) to which was added various percentages of the binder dioctyl sebacate (DOS). The ultrafine material particle size was less than 1 mum, while the conventional was 400 mum. The first aim of these experiments was to investigate the effect on the deformation, ignition behavior and impact sensitivity when DOS was added. A further aim was to determine possible sources of ignition in this energetic system. Techniques used included optical microscopy, heat sensitive film, and high-speed photography. The DOS was found to coat the individual particles of RDX. However, under impact loading, the DOS produced a blocky texture of millimeter dimensions. When ignition took place, the primary mechanism was localized adiabatic shear banding. (C) 2003 The Combustion Institute. All rights reserved.