Combustion of ash rich biomass fuels like forest residues and wheat straw often cause severe fouling/deposits and high emissions of PM1, mainly related to alkali transformation. Due to technical and air pollution aspects, primary process/fuel related measures for reduction of volatilized alkali could therefore be of importance. Peat has been used extensively in e g Sweden and Finland since the early 80th due to its positive ash chemical effects. Earlier research with co-combustion of peat and biomass has mostly been focused on fluidized bed boilers and aspects of bed agglomeration and deposits/corrosion. It has also been shown that the content and form of ash forming elements in different peats can vary significantly. The objective with this work was to determine the potential reduction of fine particle- and deposit forming alkali during co-combustion of forest residue and wheat straw with four different peat types in a small scale (15 kW) grate fired pellet boiler. The results showed that significant reduction of fine particle- and deposit forming alkali is possible, either simply by "dilution" of K content (e.g for wheat straw) or by "capturing" of K to bottom ash/slag (e.g. for forest residues), most probably caused by reaction of K vapour from the biomass with reactive Si or clay minerals from the peat. The alkali reduction potential for different biomass fuels and peat mixtures is dicussed in relation to the slagging tendencies and general ash transformation processes