Solids flow dynamics in gas-solid risers is inherently complex. Model refinement through experimental validation requires the acquisition of detailed nonintrusive measurements. In this study, noninvasive computer-automated radioactive particle tracking (CARPT) is employed to visualize and quantify in a three-dimensional domain the solids dynamics and mixing in gas-solid risers. This technique has the added advantage that, along with the derived Eulerian solids flow field (time-average velocity map and various turbulence parameters such as the Reynolds stresses, turbulent kinetic energy), it also provides directly the Lagrangian description of the solids motion. The solids velocity field data are obtained in two different risers at low and high solids fluxes at varying superficial gas velocity to span both the fast-fluidized (FF) and dilute phase transport (DPT) regimes. The effect of operating conditions on solids flow and mixing is studied. Comparative analysis of the results is presented to provide insights into the complex solids flow patterns characteristic of gas-solid risers.