Abstract: The invention relates to a process for the conversion of corrinoids produced by microorganisms into cyanocorrinoids by reaction with cyanides. According to the invention a fermentation broth obtained by disruption of microorganism cells in a known manner, preferably by heat treatment in the presence or absence of sulfite ions, optionally after purification steps known per se, is contacted with a suitable adsorbent or ion exchange resin, the corrinoids adsorbed on the surface of the adsorbent or ion exchange resin are treated with an aqueous solution containing cyanide ions or a compound capable of supplying cyanide ions, in an amount providing cyanide ions in a 1.1 to 2.0-fold molar excess related to the corrinoids, the adsorbent or ion exchange resin is washed with water, and the cyanocorrinoids obtained is eluted in a known manner, preferably with aqueous ethanol.