Flowing solids residence time distribution in gas-flowing solids-fixed bed contactors was investigated using a tracer technique with a step signal as input. Particles of different color (all other properties being the same) were used as tracers, and the residence time distribution (RTD) was obtained by the color analysis of the outlet. In additional experiments, on the basis of the determination of the tracer fraction in static holdup, the existence of exchange between static and dynamic holdup was proven. The fraction of static particles is not subjected to exchange and represents the dead part of the flowing solids in static holdup. A mathematical model which takes into account both axial mixing and exchange between static and dynamic holdup was developed. The model, based on nonideally mixed units in series, gives reasonable agreement with experimental results.