Limited availability of sawdust and cutter shavings and an increasing demand for wood pellets in Sweden are pushing the market towards new, more problematic, raw materials with higher ash content. Examples of such raw materials are bark, logging residues and whole tree assortments. The objectives of the present work were to determine the effects of kaolin and limestone addition on the slagging tendencies of some problematic woody biomass pellets during combustion in a typical residential pellet burner and also to contribute to the understanding of the role of kaolin and limestone in preventing slagging. This was accomplished by studying the bottom ash and the slag composition after 19 suspension with additive-to-fuel ratio of 1 wt %d.s. to the severely contaminated bark and cleaning assortment ignificantly reduced the slagging tendencies. Significantly higher Ca- and Al-concentrations in the slag were observed for limestone and kaolin addition, respectively. These changes resulted in a more favorable melting behavior (i.e. a igher solid phase to melt ratio).