Bed agglomeration is one of the most common problems in FB boilers. In this work the dependence of agglomeration tendency of peat and biomass ash on the content of amorphous material in ash has been investigated. The factors affecting the interactions between quartz bed particles and ash-forming elements have been modeled by involving mixtures of oxides that are commonly found in ash by utilizing X-ray powder diffraction for a semiquantitative determination of amorphous phases and SEM-EDS for compositional distributions. It is well-known that high iron or alkali metal contents increase the agglomeration tendency of ash. It can indeed be seen that the amount of amorphous material in ash increases with increasing iron or alkali metal contents. The results have been compared with those of two standard peat ashes, one of which is known to be unproblematic, while severe agglomeration has been observed in the case of the other.