To develop pretreatment strategies for better industrial utilization of biomass materials, six types of biomass were washed with deionized water at 303, 333, and 363 K, and the leachate was characterized by chemical, electrochemical, and spectral analysis. The results show that K+ is the most abundant cation in the leachates. An increase in the washing temperature leads to an increase in the cation concentration mainly because of the increment of K+. The chemical oxygen demand (COD) and the charge difference between inorganic cations and anions for leachate suggest that, in addition to inorganic ions, a few organic compounds and organic anions are released from biomass during washing. Fourier transform infrared (FTIR) spectra of the dry leachate samples reveal that carbohydrates and carboxylates are the major components of the organic compounds and organic salts, respectively. Except for the leachate of rice straw, the charge difference and COD increase with increasing washing temperature because of the increment of carboxylates for all of the other leachates.