The Cornforth reagent is prepared by slow addition of a concentrated aqueous solution of chromium trioxide to pyridine. The reaction may cause explosion, which is avoided by thoroughly dissolving the trioxide in water and cooling the solution by ice. The product is filtered, washed with acetone and dried, yielding an orange powder. The powder is stable in air, not particularly hygroscopic and has an almost neutral pH that facilitates its handling; it is only slightly acidic owing to the presence of pyridinium cations. The Cornforth reagent is readily soluble in water, dimethylformamide and dimethyl sulfoxide (DMSO). It is poorly soluble in acetone and chlorinated organic solvents, such as dichloromethane, and forms suspensions.