This work is concerned with the structure of moving beds, in particular the steady-state dense phase downwards flow of monosized particles in a vertical cylindrical vessel. The structure of a moving bed is probed using gas flows through a 'frozen' bed, which is a moving bed which has been carefully stopped to preserve its structure. The gas flows are exploited in two ways, (i) by injecting a pulse of CO2 tracer into a steady inlet flow of air, and measuring ils concentration in the outlet air. thus measuring thp gas residence time distribution (RTD), and (ii) by injecting a pulse of CO2 at a particular position in the bed, and measuring its concentration at a position vertically above the injection point, thus measuring the gas velocity at a particular radial position in the bed; a radial traverse then yields a radial profile of tracer gas velocity. The concentration of the CO2 tracer is measured using a mass spectrometer. Separate experiments using glass spheres of diameter 1, 3 and 5 mm in a bed of diameter 0.14 m show significant gas flow maldistribution, in both RTD and radial profile of tracer gas velocity. The level of maldistribution depends crucially on the frictional properties of the wall of the vessel. When the wall of the vessel is smooth, the RTD has the Gaussian shape characteristic of axially dispersed plug flow, and the radial profile of tracer gas velocity is flat; when the wall of the vessel is rough, the RTD is wide and asymmetric, and the gas flow maldistribution is shown directly in the radial profile of tracer gas velocity. Limited results are also presented for polypropylene spheres and cylinders, and steel spheres. The measured RTD, with maldistribution, is successfully described by modelling the gas flow using axially dispersed plug flow with no radial mixing but taking into account the measured gas velocity variation over a bed cross section, together with end zones each consisting of a well-mixed volume and a dead volume.