Biomass boiler issues regarding slagging, fouling and corrosion are related to alkali species present in fuels. These alkali species are released as gaseous alkali chlorides, hydroxides and/or sulfates during combustion. Alkali chlorides/sulfates later condense on cold boiler surfaces enhancing fouling and corrosion. Subsequent deposition of silica-rich ashes leads to the formation of low temperature melting eutectics, especially alkali/earth alkali-silicates (K/Ca-Si), and creates compact and strong deposits. Condensing chlorides/sulfates and low melting eutectics both reduce boiler performance and its availability. Two different woody biomass fuels and their mixture with two different kinds of aluminosilicate base additives were combusted during this study. Deposits were sampled using two different kinds of probes; cooled deposit probe and un-cooled deposit probe. The macroscopic and microscopic observations and chemical composition of deposit samples are compared using electron microscopic image analysis (EMPA) for both deposit samples. The additive significantly changes the morphology and composition of the deposit samples. The difference in morphology and composition can be explained by the change in potassium chemistry between two cases. (C) 2015 Elsevier B.V. All rights reserved.