Agglomeration of bed material and fuel ash may cause problems during fluidized bed combustion of biomass fuels. Previous results have shown that a "sticky" coating, which covered the original bed material and consisted of Ca-K-silicates, was directly responsible for the bed agglomeration during biomass combustion. The melting behavior (stickiness) of these coatings was very sensitive to the potassium content. Frier studies have also indicated that bed agglomeration could possibly be prevented by introducing low-cost additives such as kaolinite. The objectives of the present work were, therefore, to illustrate the effect of kaolin addition on the actual agglomeration temperature of two troublesome biomass fuels, and to contribute to the understanding of the role of kaolin in prevention of bed agglomeration. By controlled agglomeration experiments in a 5 kW bench scale fluidized bed reactor, the critical temperatures for agglomeration In a normal quartz bed when firing wheat straw or bark were determined to be 739 and 988 degrees C, respectively. By adding kaolin, 10% w/w of the total amount of the bed, the initial bed agglomeration temperatures increased to 886 and 1000 degrees C, respectively. Samples of bed materials, collected throughout the experimental runs, as well as final agglomerates were analyzed using SEM/EDS and X-ray diffraction. These results showed that kaolin was transformed to meta-kaolinite particles, which adsorbed potassium species. The increased agglomeration temperature was explained by the decreased fraction of melt in the bed particle coatings, i.e., coatings were somewhat depleted in the potassium content by the corresponding potassium-enrichment in the kaolin-derived aggregates.