We report the use of carboxyl‐functionalised alkylimidazolium salts as thermomorphic acid catalysts for the hydrolysis of cellulose and starch in water, free from organic solvents and auxiliary substances. The imidazolium salts are insoluble in water at room temperature and dissolve to form homogeneous solutions upon heating. Following catalysis at elevated temperatures the solution is cooled and the imidazolium salt precipitates from the aqueous layer to afford an aqueous glucose solution. This thermomorphic, temperature‐switchable, process allows facile separation of the product and reuse of the catalyst while ensuring catalysis commensurate with homogeneous systems.