An investigation was made of the phenomenon of defluidisation caused by visco-plastic sintering. This occurs when the fluidised bed is operated at a temperature sufficiently high for material at the surface of the particles to flow. Previous theoretical treatments of the problem have employed force and energy balances to express the ability of sintered aggregates to withstand the hydrodynamic forces imposed by the fluidised bed. A new conceptual approach is proposed which is based on a comparison of the characteristic times for quiescent motion of particles in a bed and for the growth of sinter necks. The theory gives a quantitative relationship between the gas velocity required to maintain fluidisation and the temperature of the bed in terms of the surface viscosity of the particles. Experimental results are reported on the defluidisation and sintering behaviour of a model material (low density poly-ethylene). The data are used to test the theory and to evaluate the potential of the new approach.