TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: The inorganic reducing agent is phosphonic acid, sodium bisulfite or sodium dithionite. The dibromine is present at 1-1.5 molar equivalents. The cysteine derivative (IV) is present at 2-10, preferably 3-7 molar equivalents.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Process: The preparation of (I) from compound (III) is performed in a single reactor without isolation of intermediate (II). (II) is prepared from an acid addition salt, except hydriodic acid salt, of substituted betaine compound of formula (III) and their tautomers, or stereoisomers or its mixtures in all proportions, preferably mixture of enantiomers, including racemic mixtures, by successive reaction with dibromine, and then with a cysteine derivative of formula (HS-CH2-CH(NHR6)-C(=O)-OR7) (IV) or its stereoisomer or its mixtures, preferably a mixture of enantiomers, including a racemic mixture. Preferred Components: The organic reducing agent is: a thiol compound of formula (A0-SH); and a phosphine compound of formula (PA1A2A3). The thiol compound is cysteine, dithiothreitol, 2-mercaptoethanol, 2-mercaptopropionic acid, 3-mercaptopropionic acid (preferred) and thioglycolic acid.A0 = 2-6C, preferably 2-4C alkyl chain substituted by one or more groups comprising OH, SH, NH2 or COOH; andA1-A3 = 2-6C, preferably 2-4C alkyl chain substituted by one or more OH, SH, NH2 or COOH. SPECIFIC COMPOUNDS - Preparation of L- ergothioneine of formula (Ia) is specifically claimed as (I).    EXAMPLE - 2-(2-((2-Ammonio-2-carboxyethyl)thio)-1H-imidazol-4-yl)-1-carboxy-N,N,N-trimethylethanaminium dichloride dihydrochloride (1.67 g) was dissolved in water (16.7 ml), and 3-mercaptopropionic acid (1.895 ml) was added. The mixture was heated with stirring for 24 hours at 85 degrees C. Then 3-mercaptopropionic acid (1.895 ml) was added and heating was continued for 48 hours. The reaction mixture was cooled to 0 degrees C and a white precipitate was formed. After filtration, the precipitate was washed with cold water (2x 2 ml), the filtrate was washed with dichloromethane (5x 20 ml) and ethyl acetate (5x 20 ml). The aqueous phase was evaporated under vacuum and the residue was dissolved in water (33 ml). The pH of the solution was adjusted to 6, by adding a solution of 20% ammonia. After lyophilization, the resulting powder was dissolved in hot ethanol/water (7/1) mixture. After addition of activated charcoal (100 mg) and hot filtration, the resulting solution was kept at room temperature for 14 hours. After filtration and drying, L-ergothioneine (676 mg, 69%) was obtained.