Pretreatment breaks the recalcitrant structure of lignocellulosic biomass prior to enzymatic hydrolysis and fermentation for the production of biofuels and chemicals. In this study, Miscanthus × giganteus, a C4 perennial grass and energy crop, was pretreated effectively with a new one-step chemical approach at 90 °C–110 °C for 6–30 h using switchable butadiene sulfone. The solvent has the ability to “switch” in equilibrium to 1,3-butadiene and sulfur dioxide, forming sulfurous acid in the presence of water. Sulfurous acid hydrolyzed xylan in Miscanthus, removing up to 91% of xylan into the liquid phase, via Brønsted acid catalysis following irreversible first-order kinetics with the activation energy determined to be 89 kJ mol−1. Butadiene sulfone was mainly responsible for the solubilization of up to 58% of lignin during or after the hydrolysis of xylan. X-ray diffractograms showed that unwanted diene polymers from solvent decomposition can be minimized in the pretreated solids below 110 °C and 18 h, conditions that also allowed for the preservation of 90–99% of glucan. It was demonstrated that all of the butadiene sulfone after pretreatment can be decomposed into 1,3-butadiene and sulfur dioxide, which enables potential solvent reformation and separation from the solubilized xylan derivatives and lignin.