In this paper a mathematical model for solids flow in gas-flowing solids-fixed bed contactors is developed. The presented four-parameter model assumes plug flow with axial dispersion in the dynamic, fast moving solids zone and the exchange of particles from this zone with the static zones. The complex dynamic behavior of the 'static' particles is described with a model of exchange between the dynamic and three static zones: (a) the 'fast' exchanging, (b) the 'slow' exchanging and (c) the 'dead' zone. The model parameters are optimized on the basis of two different tracer experiments: the step change response at the outlet-chi(dyn), and the static holdup response-chi(st). The model was tested for seven experimental series which correspond to different operating conditions (e.g.. different superficial gas velocity and solids mass flux). The influence of the operating conditions on the model parameters is presented and discussed. Model reduction was also implemented in order to analyze the model simplifications and alternatives. Model sensitivity analysis was performed as well. (C) 2009 Elsevier Ltd. All rights reserved.