One is, however, justified in assuming that both the natural tannins and the related humic acids are ester-derivatives of hydroxybenzoic acids.

Amongst the small number of synthetically prepared ester anhydrides of hydroxybenzoic acids, a few occur exhibiting the properties of the natural tannoids.

This simple procedure, however, is not applicable to most other hydroxybenzoic acids which are decomposed at the high temperature necessary to induce reaction.