The hydrodynamic and mixing behavior of binary solids mixture spouted beds with particles of the same size but different densities have been investigated experimentally in gas-solid spouted beds for the first time, using an advanced non-invasive radioactive particle tracking (RPT) technique. The RPT experiments have been performed for different composition of binary mixture at different superficial gas velocity. The binary solids mixture used in the experiment consists of glass beads and steel particles of densities 2500 and 7400 kg/m(3), respectively. The RPT experiments were carried out for each one of the solids phases on an individual basis, and then the statistical averages of each flow fields are combined together to formulate the entire picture of the binary solids mixture flow field. It was found that segregation always takes place in the spout due to the dissimilar behavior between the different solids phases in terms of the solids velocity field and turbulent parameters. The results also demonstrated that for the hydrodynamics of binary solids mixture spouted beds which have particles of similar size but different densities, the particle-particle interaction plays an important role, but it does not dominate the gas-particle interaction, and each contributes to the extent of the mixing and segregation phenomena inside the bed. The results and findings of our work are valuable in understanding the hydrodynamics of the binary solids mixture encountered in the gas-solid spouted beds of TRISO nuclear fuel particles and provide benchmark data to validate computational fluid dynamics (CFD). (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.