The effect of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO) and two of its transition metal salts, namely Cu(NTO)(2) and Fe(NTO)(3), during the combustion of composite solid propellants (CSPs) of hydroxyl-terminated polybutadiene (HTPB) and ammonium perchlorate (AP) has been studied. The activities of Cu(NTO)(2) and Fe(NTO)(3) have been compared with those of CuO and Fe2O3 at their equivalent metal concentrations. The processing parameters and mechanical properties of the propellants were also evaluated using Cu(NTO)(2) and Fe(NTO)(3) as additives, and a comparison has been made with that of copper chromite (CC) and active iron oxide (AIO) at pilot plant scale. The safety aspects of using these energetic salts as burning-rate modifiers have been studied in terms of the impact-sensitivity of the modified propellants. An attempt has-been made to evaluate experimentally the condensed phase activity of these additives during the slow thermolysis of propellants, as well as AP. Rapid thermolysis of the propellants and AP has been studied using measurements of ignition delay. (C) 2003 The Combustion Institute. All rights reserved.