Fast circulating fluidized-bed reactors are used for many applications, both in riser and downer modes. Typical residence times are in the order of a few hundreds of milliseconds. A good understanding of initial gas-solids contact must be reached to realize the full potential of such reactors. Effective and stable gas-solids mixing chambers were identified from the signals of simple and robust probes, which could be used in industrial and pilot plants. Criteria based on cross con elation or Hurst analysis provided consistent results. Although this study wed simplified momentum probes, the criteria it developed can be applied to the signal of any fast-response sensor which is affected by local hydrodynamics. Temperature, capacitance, ultrasonic, or gamma-ray absorption measurements could be used. Gas or liquid-liquid mixing chambers could also be optimized with the same criteria.