The decomposition and combustion behaviour of ammonium perchlorate (AP) and simple bi-propellant systems were studied. Three types of fuel binders were used in order to examine the role of binder on the combustion process. Paraffin wax was chosen as a simple model fuel, and AP/3-9% (mass%) paraffin wax mixtures were used. It was observed that the paraffin melted and volatilized to react with AP decomposition products in the gas phase. The same behaviour was observed for AP/polyvinyl chloride propellant. In contrast, hydroxy terminated polybutadiene (HTPB) undergoes condensed phase reactions before the reaction with AP occurs. In agreement with the kinetic data, the condensed-phase heat release was higher for AP/paraffin combustion than that of AP/ HTPB propellant. However, a lower burning rate was observed for the paraffin propellant and evidence is given that the role of gas-phase reactions is more pronounced even at atmospheric pressure.