Agricultural residues and energy crops often contain high concentrations of alkali metals, chlorine, and other elements that promote ash slagging, heat exchanger fouling, and corrosion in thermochemical conversion systems for biofuel production and electricity generation. Leaching was investigated for improving fuel quality of rice and wheat straw, corn stover, switchgrass, and Miscanthus by reducing concentrations of water soluble alkali metals, chlorine, and sulfur in biomass. Samples were leached in water at ambient temperature at solid-to-liquid ratios of 1/20 kg L^-1. Leachates were analyzed via HPLC for selected inorganic and organic constituents. Leaching removed 54 - 66% of sodium, 51 - 82% of potassium, 79 - 95% of chlorine, and 42 - >99% of sulfur. Total sugars, acids, and glycerol extracted from the feedstocks were 7 - 103, 9 - 58, and 0.3 - 1.6 mg g^-1 dry matter respectively. Ash fusion temperatures increase for rice straw from 1050°C to 1550°C, wheat straw from 900°C to 1250°C, switchgrass from 900°C to 1050°C, and Miscanthus from 1150°C to 1250°C; however, no substantial change was observed in fusion temperature for leached corn stover due to the compositional shift being confined to the low melting point range. Leaching is beneficial as a feedstock pretreatment for improving thermochemical properties although starting compositions need to be considered in designing leaching conditions to realize improvements in ash fusibility. Sugar and acid extracts may be produced as value-added co-products.