Under relatively mild conditions (≤140 °C, 1 atm) and in the absence of added acid catalysts typically employed in biomass conversion, cellulose dissolved in certain ionic liquids (ILs) has been converted into water-soluble reducing sugars in high total reducing sugar yield (up to 97%), or directly into the biomass platform chemical 5-hydroxymethyl furfural (HMF) in high conversion (up to 89%) when CrCl2 is added. The combined study of experimental methods and ab initio calculations demonstrates that the significantly increased Kw by ILs in the IL−water mixture is responsible for the catalysis seen in the current efficient biomass conversion system without added acid catalysts. The finding that the water in ILs under mild conditions can exhibit high Kw values (up to 3 orders of magnitude higher than the pure water under ambient conditions) is significant because such high Kw values are typically achievable by the water under harsh high-temperature or subcritical water conditions.