Industrial fluidized beds are operated with particles that usually contain impurities or are themselves a mixture of many components (for example, ores). Upon heating to high temperatures, some of these components soften, melt or react with each other, generating stickiness. In fluid bed reactors, the fluidizing gas can also react chemically with the solids producing new components and during this process particle cohesion and stickiness can occur. In all these cases, the fluidization behavior of the particles changes dramatically as temperature is increased, agglomerates form and such characteristics as the minimum fluidization and bubbling velocity, the bubble size and bubble frequency, etc., all change significantly. It has been demonstrated in earlier work by the present authors and also by others that above a certain temperature (the so-called sintering point), fluidization is not possible at all unless special precautions are taken and the gas velocity is increased significantly. The goal of the present research is to determine the sintering or ’sticking’ temperature in these more complicated cases when the particles are chemically complex and/or when chemical reactions take place. It is shown during the present work that even under these conditions, sintering temperatures can be measured using the dilatometer technique developed earlier for inert gases and pure materials assuming that such conditions as gaseous atmosphere and temperature can be reproduced in the instrument. A special class of agglomeration due exclusively to the formation of new species during a chemical reaction which occurs on the surface of the solid particle is also presented in this paper. It is shown that at temperatures well below the softening (sintering) points of both the reactants and the products, particle agglomeration can occur during the process of product formation : examples such as the oxidation of coke and magnesium powders, the reduction of calcium sulfate and the production of aluminum nitride are given.