One of the main concerns about biomass fluidized-bed gasification and combustion is the risk of bed particle agglomeration because of ash melting. Although many studies have been conducted about the agglomeration mechanism using silica sand, olivine is mostly mentioned as an alternative bed material for tar decomposition, and its interaction with biomass ash has not yet been fully understood. The aim of this work is to investigate the agglomeration of Miscanthus ashes, focusing on thermophysical and thermochemical aspects. Three different bed materials {silica sand (SiO2) and raw and calcined olivine [(Mg,Fe)(2)SiO4]} and an additive to prevent agglomeration {dolomite [CaMg(CO3)(2)]} were tested. The effects of atmosphere and Miscanthus harvest time were also investigated. It was found that the key parameter of agglomeration is the wettability of bed particles by molten ashes. In contact with ashes, all three bed materials showed good wetting tendencies, while dolomite had non-wetting properties. The adhesion between bed materials and molten ashes increases in the order of silica, olivine, and calcined olivine. While, in the case of silica sand, only physical adhesion occurred, the diffusion of iron oxide into the molten ash was observed using olivine. Calcined olivine has a roughened surface, which further increased the adhesion. The atmosphere did not influence the mechanism of the ash/bed material interaction. On the other hand, Miscanthus harvest time had a significant effect on ash reactivity and interaction with raw and calcined olivine.