NOVELTY - Preparing ergothioneine compounds (I) and their salts, tautomers, or stereoisomers or its mixtures in all proportions, preferably mixture of enantiomers, including racemic mixtures, comprises: reacting betaine compounds (II) and their salts, tautomers, or stereoisomers or its mixtures in all proportions, preferably mixture of enantiomers, including racemic mixtures, with an organic or inorganic reducing agent, preferably soluble in the reaction solvent which may be water, to give (I); and separating (I) from the reaction medium.    USE - The process is useful for preparing ergothioneine.    ADVANTAGE - The process: provides (I) in good yield and at low cost; is suitable for industrial scale preparation of (I); and is environmentally friendly. The process avoids: difficulties of purification; and the use of hazardous products or solvents, which are toxic to humans and the environment.    DETAILED DESCRIPTION - Preparing ergothioneine compounds of formula (I) and their salts, tautomers, or stereoisomers or its mixtures in all proportions, preferably mixture of enantiomers, including racemic mixtures, comprises: reacting betaine compounds of formula (II) and their salts, tautomers, or stereoisomers or its mixtures in all proportions, preferably mixture of enantiomers, including racemic mixtures, with an organic or inorganic reducing agent, preferably soluble in the reaction solvent which may be water, to give (I); and separating (I) from the reaction medium.    R1, R2 = H or 1-4C alkyl such as methyl;    at least one of R1, R2 = H or preferably each R1 and R2 represents H; and    R3-R5 = 1-4C alkyl such as methyl;    R6 = H, 1-4C alkyl or -CO-(1-4C alkyl), preferably -COCH3 or H;    R7 = H or 1-4C alkyl, preferably H;    dotted bond = optional bond; and    X1 = R1N or N;    X2 = NR2 or N.    INDEPENDENT CLAIMS are included for:    (1) the betaine compound (II); and    (2) preparing (II) comprising: reaction of an acid addition salt, except hydriodic acid salt, of substituted betaine compound of formula (III) 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, where the cysteine is L-cysteine.