To a solution of 24 g. (0.1 mole) of cystine in a cold mixture of 150 ml. of water and 50 ml. of concentrated hydrochloric acid is added, dropwise, 80 g. (25 ml., 0.5 mole) of commercial bromine, with occasional stirring, during 40 minutes. The temperature of the mixture rises to about 60°. The resulting solution, which contains a little unreduced bromine, is then evaporated under reduced pressure on a steam bath. The dark-colored crystalline residue is dissolved in 100 ml. of distilled water and filtered from a small quantity of amorphous insoluble matter. The filtrate is concentrated by evaporation on a water bath to 65 ml. and allowed to crystallize by standing overnight in a refrigerator. The crystals are filtered with suction and washed well with about 100 ml. of 95% ethanol in several portions, the washings being collected separately. The crystals are dried under reduced pressure over phosphorus pentoxide. A second crop is obtained by diluting the washings with an equal volume of water, evaporating until free of ethanol (Note 1), adding the residue to the mother liquor, and evaporating the combined solution to dryness on the water bath. The residue is dissolved in 30–40 ml. of water, decolorized with 0.5–1.0 g. of charcoal, concentrated to 15 ml., and, when cold, treated with 30 ml. of 95% ethanol. The crystals so formed are collected, washed with ethanol, and dried as before (Note 2). The total yield is 30.5–33.5 g. of pure cysteic acid monohydrate (81–90%). It melts, with vigorous evolution of gas, at 278° (289° cor.) (Note 3), and shows the rotation [α]₅₄246 + 9.36° (6% in water).