A process for the preparation of N-(D)-ribityl-2-phenylazo-4,5-dimethylaniline (I), wherein1. in the case of pure or virtually pure (D)-ribose (III)(a) the latter is reacted with 3,4-dimethylnitrobenzene (IVa) or 3,4-dimethylaniline (IVb) and with hydrogen in the presence of a hydrogenation catalyst,(b) the resulting solution is reacted, in a conventional manner, with an acid phenyldiazonium salt solution (VI) and(c) the resulting product is isolated by crystallization, in a conventional manner, or2. in the case of crude ribose, ie. industrial mixtures of (D)-ribose and other sugars(a) the crude ribose is reacted with about equimolar amounts, based on III, of 3,4-dimethylaniline (IVb) and boric acid,(b) the boric acid ester of the Schiff base obtained from III and IVb is allowed to crystallize out and is separated off,(c) this ester is hydrogenated with hydrogen in the presence of a hydrogenation catalyst,(d) the solution is freed from catalyst and reacted, in a conventional manner, with an acid phenyldiazonium salt solution and(e) the resulting product I is isolated by crystallization in a conventional manner.