Clear definition of smooth and stable operating regimes is necessary if circulating fluid bed reactors are going to be adopted commercially for process applications that require load following capabilities. One significant source of instability or long-term transient occurs when traversing the transition from transporting in a lean phase fluidization regime to transporting in a turbulent or fast-fluidized bed regime. The gas saturation carrying capacity (SCC) is the key functional parameter required to define this transition. A simple method is presented to accurately determine the SCC in which the solids flux was abruptly halted and the loss in pressure signal monitored over time. The SCC was measured for PVC under a variety of conditions. Upon drawing from the analogous liquid-vapor to vapor phase transition, a predictive relationship is also presented to estimate SCC for various gas-solid flow systems, knowing the gas velocity and solids' terminal velocity in that gas. Published by Elsevier Science B.V.