In this report, 1-butyl-3-methylimidazolium chloride ([C4C1im]Cl) is demonstrated to enhance the kinetics of acid-catalysed hydrolysis of 1,4-β-glucans in binary solvent mixtures. [C4C1im]Cl plays other roles in the reaction beyond acting as a solvent for cellulose, as currently accepted. In fact, the presence of the IL increases the Hammett acidity of the catalyst dissolved in the reaction medium. The kinetic data from cellobiose and cellulose hydrolysis directly correlate with the acid strength found for p-toluenesulfonic acid in the different reaction media studied here. The current report identifies neglected, but yet very important phenomena occurring in cellulose depolymerisation.