Investigations have been ongoing to learn the rheological and/or mechanical behavior of composite solid propellants based on hydroxyl-terminated polybutadiene (HTPB). The mechanical properties of these materials are related to the macromolecular structure of the binder as well as to the content and nature of the fillers. The viscoelastic behavior of an HTPB binder and its composites with different types of fillers was surveyed by dynamic mechanical analysis over a wide range of temperatures. This technique has clearly demonstrated a two-phase morphology developed in these systems. The temperature location, intensity, and apparent activation energy of the distinct relaxations are discussed. The dependency of the relaxation processes on filler content in a series of composites has elucidated the interactions between the filler particles and the existing hard- and soft-segment domains within the polyurethane matrix. It was observed that the nature of the filler significantly affects the relaxation process associated with the hard-segment domains of the polymeric structure. (C) 2003 Wiley Periodicals, Inc.