Pyroclastic density currents (PDCs) are particle-laden, gravity-driven flows of volcanic origin. They typically contain solid particles with a wide range of sizes and densities, resulting in complicated segregation effects during transport and sedimentation. This paper describes experimental high-density aqueous suspension flows scaled approximately to turbulent pyroclastic density currents. It provides information on the development of concentration and particle size gradients in such flows and their relationships to vertical and stream-wise grading in the resulting deposit. The currents were generated by lock-exchange release of initially homogeneous, suspensions of highly polydispersed particles with concentrations of 13.8–23.0 vol.%. Size distributions were chosen to encompass a wide range of Rouse ( Pn) numbers, from particles that were fully supported by turbulence in the currents ( Pn《2.5) to those whose motion was essentially decoupled from turbulent eddies ( Pn》2.5), as in many pyroclastic density currents. Experimental currents of two different particle populations were studied: one containing particles of a single density, and another with particles of two different densities. In each case, the current head was sampled at different locations along the flume in order to study the evolution of vertical gradients in particle size, concentration, and density distribution in the propagating flow. These gradients were then related to grading observed in the resulting deposits. Differential settling resulted in rapid segregation of particles of different sizes and densities in the currents and the development of both vertical and longitudinal gradients in particle concentrations, sizes and proportions. Longitudinal gradients developed as vertical gradients were pulled out longitudinally by the vertical velocity profile in the fluid. Gradient development involved principally large particles capable of moving independently of turbulent eddies in the fluid. The segregation behaviour of large, light particles (equivalent to pumice in the natural system) in the bidensity currents was strongly influenced by buoyancy forces in the dense initial suspension. Sedimentation of large, light particles was delayed and reverse vertical and longitudinal grading of these particles was generated in the resulting deposit. Reverse grading in the deposit was not caused by flotation of the large, light particles, which were normally stratified in the current during transport. Deposition of large, light particles was probably delayed by a thin boundary layer of sufficiently high bulk density at the base of the rapidly evolving density-stratified current. Vertical or longitudinal reverse grading of large pumices in pyroclastic density current deposits does not necessarily imply flotation during transport.