It is demonstrated that transfer hydrogenation from formic acid to levulinic acid under hydrothermal conditions can be catalyzed by bases, but also by simple sodium sulfate. The action of salt addition could be clarified and ascribed to the changing dissociation constants at high temperature. This renders sulfate a temperature-switchable base in hydrothermal syntheses. Using such salts can help in preventing waste, as neutralization after reaction is not necessary. By optimizing the reaction conditions, the yield of γ-valerolactone, a sustainable biofuel molecule, could be raised to 12% for a simple passage through a capillary flow reactor with a residence time of less than 20 min.