An investigation of the flow of suspensions of solids particles in gases at high flow velocities has been carried out. A literature survey into gas-solids flow at both low and high speeds has revealed that most of the existing work on high speed flows is of a theoretical nature and has been stimulated as a result of the presence of solids particles in the exhaust gases of solid-fuelled rocket engines. In this project theoretical approaches are presented for both idealised flows where the solids and gas are in equilibrium or maintaining constant fractional lag and also for the more realistic case of variable gas-solids velocity and temperature ratios where solutions require numerical integration by a digital computer. The experimental approach has been limited to the flow of polystyrene solids particles in air through a long 1 in. nominal bore pipe at flows ranging from almost incompressible to choked flows. Measurements of air and solids mass flow rates, air velocity profiles and solids velocities have been obtained. The indications are that provided an estimation of the frictional drag due to the gas skin friction and the solids particle to particle and particle to duct wall interaction can be obtained, the numerical solution gives good results. The constant fractional lag theory gives indications of providing a Mach number expression capable of predicting the choked flow of gas-solids suspensions.