an integrated process for manufacture of trimellitic acid anhydride by the steps of catalytic oxidation of pseudocumene in the presence of acetic acid in an oxidation zone wherein liquid - phase conditions are mantained , cooling the oxidation reaction effluent to crystallize trimellitic acid , separating and recovering crystallized trimellitic acid , conducting the thermal conversion of the trimellitic acid to its anhydride continuously in two series staged dehydration zones and evaporating acetic acid solvent and hydrocarbon . vapor from the first dehydration zone and the acetic acid mother liquor are combined as feed for distilling acetic acid and water mixture therefrom leaving a residue having high melting solids . the residue is combined with a bottom fraction from distillation of trimellitic anhydride product having catalyst metals and the combined material is added to the first dehydration zone in an amount of up to about 50 weight percent of the combined residue and bottom fraction .