The burning rate behaviour of a series of bimodal PETN/Polyurethane (PU) and RDX/PU propellants have been examined to determine the effect of binder and oxidizer concentration, and coarse to fine ratio of oxidizer particles on burn rate. Evidence for an oxidizer interaction has been found in mixed PETN/RDX/PU propellants. Catalysis of the PETN/PU, RDX/PU and PETN/RDX/ PU propellants by a metal oxide has been investigated. Catalytic enhancement in both the condensed phase and gas phase has been observed. Oxidizer interaction between PETN and RDX can lead to substantial increases in burning rate in the presence of catalyst.