Abstract: Optionally aldehyde substituted polyphenols are prepared by oxidizing, with hydrogen peroxide, a hydroxybenzaldehyde bearing at least one aldehyde substituent ortho- and/or para- to the nuclear hydroxyl group, in an aqueous reaction medium and in the presence of an alkali or alkaline earth metal base, the process being characterized in that the pH of the reaction medium is continuously maintained at a value no greater than 7 throughout the course of the oxidation reaction.The subject process is well suited for the preparation of, e.g., hydroxy-p-vanillin from guaiacol, and the novel compound 2,4,6-triformylphenol.