To deepen our knowledge of the flow in cyclones, flow velocity and local solids concentration measurement techniques were developed. The flow consists of a high mass concentration gas-solid suspension (up to 2 kg of material per kilogram of gas) and a vortex generated by the cyclone. Sampling is a direct and simple method which does not require any calibration and enables to carry out chemical and grain size analyses on the collected samples. The descriptions of the probe used and of the testing procedure are followed by the presentation of results obtained in a cyclone. An analysis has permitted to characterize the inlet conditions of the suspension in the cyclone and to identify the characteristics of the vortex flow. Inlet concentration increases linearly with the inlet height and three structures are detected: a 'dense phase', material in excess dumped to the cyclone wall as soon as it enters, a direct escape of material from the inlet to the gas outlet and a suspension, material submitted to the vortex centrifugation. Investigation of the efficiency per grain size range allows quantifying the phenomena revealed by the solids concentration measurements. Material in suspension in the cyclone, composed by escape flow and suspension, called 'critical load' by Muschelknautz [13], increases linearly with the inlet loading rate until the material saturation of the vortex. The structure of the flow, identified owing to the experimental measurements, is closely dependent on cyclone geometry. This work was realized in the Research Center of FCB, manufacturer in cement industry