The interchange reaction of the three phthalic acid isomers with triglyceride oils at high temperatures was studied. Orthophthalic acid dehydrates to the anhydride before any interchange occurs, but isophthalic and terephthalic acids are heat-stable and react easily with oils in the temperature range of 260–300°C. Isophthalic acid reacts more rapidly than terephthalic acid and is ideal for use in acidolysis processes of alkyd-resin manufacture. Various methods of application of the acidolysis reaction to isophthalic alkyd resin preparation were studied, and the minimum of oil conversion to monobasic acids that is required for successful alkyd preparation was found. Under certain conditions the acidolysis reaction occurs simultaneously with esterification; use is made of this fact to improve the process. Commercial use of the acidolysis reaction in alkyd-resin manufacture is economically attractive because it is noncatalytic; therefore no catalyst residues need to be removed by pressure filtration. Another advantage is that glycerol is readily pumped into hot acidolysis-reaction mixtures without opening the kettle, and this eliminates the disagreeable and toxic fumes that escape from hot kettles when they are opened after alcoholysis for addition of solid phthalic anhydride or isophthalic ac