Cellulose is the most widely distributed source of biomass, and its efficient conversion to a variety of chemicals is important for a sustainable future. In this work, sulfur dioxide (SO2) dissolved in hot water has been demonstrated to be an efficient catalyst for the selective conversion of cellulose to chemicals such as glucose and levulinic acid. The selectivity of products can be tuned by the SO2 concentration, temperature, and reaction time. SO2 acts both as a supply of H+ ions through ionization of H2SO3 when dissolved in water and as a Lewis acid catalyst that breaks the hydrogen bonds in cellulose. Importantly, SO2 in the reaction mixture can be recovered completely by stream stripping, thus avoiding the formation of acidic wastewater. This work provides a new, efficient, and environmentally benign way to convert cellulose to chemicals.