This paper presents results from systematic agglomeration experiments in a straw-fired laboratory-scale fluidized bed combustor and a theoretical study of the phenomena. Experiments were carried out at different operating conditions. Defluidization resulting from agglomeration occurred in all experiments. The agglomeration tendency is represented by the time before defluidization is detected. The results show that the temperature has a pronounced effect on the defluidization time, which can be significantly extended with decreasing temperature. Examination by various analytical techniques of the agglomerates sampled during combustion suggests that the high potassium content in straw causes the formation of agglomerates and eventually defluidization. In the combustion process, potassium-containing compounds are prone to remain in the bed and form low melting potassium-rich ash. The molten ashes coat the surfaces of the bed material, promoting agglomeration and defluidization in FBC. Thermodynamic equilibrium calculations have been performed to identify the stable silica, potassium, chlorine and sulphur species. The results show that potassium silicates are the main form present in the bed, which agree qualitatively with the experimental results. Based on a competition between the strengthening adhesive force by the ash coating and sintering processes and the breaking force induced by bubbles in the bed, a simple model has been developed to describe the defluidization time as a function of temperature, fluidization velocity and particle size. The results are in good agreement with experimental results. (C) 2003 Elsevier B.V. All rights reserved.