The type of transverse bed motion in rotary cylinders depends on the Froude number and the ratio of the cylinder to particle diameter. In the case of the "rolling bed", the radial velocity profile can be obtained by modelling the bed as a viscous-inelastic, dilatant fluid. In comparison with experimental data on the thickness of the so-called active layer, this model shows good agreement. The convective mass transfer coefficient at the surface of the bed generally depends on the axial gas velocity, particle size, rotational speed and cylinder geometry. However, the influence of the rotational speed can be reduced to the transverse particle velocity at the surface. Experimental data obtained from drying experiments allow a general correlation to be fitted for the heat and mass transfer at the surface of the bed.