The dependence of tartaric acid production by Gluconobacter oxydans ssp. oxydans ATCC 19357 and G. oxydans ssp. suboxydans ATCC 621 on vanadate was investigated. It was found with both organisms that trataric acid could only be produced in a medium containing vanadate (NH4VO3). A proposed intermediate of the tartaric acid metabolism in G. oxydans, 5‐ketogluconic acid, was tested on its reactivity in the presence of the oxidizing catalyst vanadate. It could be shown that 5‐ketogluconic acid and the catalyst vanadate, but not the activity of G. oxydans, were responsible for the formation of tartaric acid. G. oxydans was not able to produce tartaric acid by itself. The stereochemical identity of the formed tartaric acid could be identified as the L‐(+)‐type. Oxalic acid was formed from 5‐ketogluconic acid with vanadate in the absence and in the presence of G. oxydans. The ratio of oxalic acid to tartaric acid was 1:1.