Agricultural residues (wheat straw, olive husk, sunflower stalk, rice straw, and tobacco stalk), especially wheat straw, may have high contents of chlorine and potassium, elements that are very undesirable in power plant fuels. Chlorine, which is found in certain biomass types, such as straw, may affect operation by corrosion. The high chlorine and alkali content of some biomass fuels raise concerns regarding corrosion. The aim of this study was to determine elemental concentrations of alkalis and chlorine in fresh biomass char and ash and their leaching behavior under a variety of leaching media. A pretreatment process to remove potassium from agricultural residues may be based on pyrolysis followed by char leaching. The residues are pyrolyzed at 798 K, at which point the potassium is retained in the char. Potassium and residual chlorine are leached from the residual char by water. Demineralization of the agricultural residue chars was investigated, and an average of 71, 72, and 98% of the K, Cl, and Na in chars were leached by washing with water at 355 K.