A process for the manufacture of o-nitrobenzaldehyde by oxidation of o-nitrotoluene, characterized in that o-nitrotoluene is made to react with an aqueous solution containing Cerium.sup.iv perchlorate and from 5 to 9 moles/liter of perchloric acid; according to a preferred embodiment, the dispersion of o-nitrotoluene in an aqueous solution containing from 5 to 9 moles per liter of perchloric acid is first prepared and then an aqueous solution is gradually added, containing Cerium.sup.iv perchlorate and 5-9 moles/liter of perchloric acid.