Tricarboxylic acids are prepared by reacting unsaturated mono- and dicarboxylic acids of specific carbon atom content in the presence of a catalytic proportion of an unactivated clay and water. The product acids of 34, 35, 36, 38, or 40 carbon atoms are useful as corrosion inhibitors, epoxy resins curing agents, lubricants, polymeric resin plasticizers and intermediates for synthetic polymeric resins.