A process for producing alkenyl substituted anhydrides which comprises introducing one mole of polypropene or polybutene having a number average molecular weight of 200-3000, 0.8-5.0 moles of the anhydride of an unsaturated dicarboxylic acid and 5-200 ppm of an additive which suppresses the formation of tarry materials and undesirable reaction side products and/or improves yield, into a reaction zone, reacting said materials at a temperature from about 150.degree. C to about 300.degree. C to form the alkenyl anhydride, distilling off and condensing the unreacted anhydride from the reaction mixture, recovering the alkenyl substituted anhydride, and returning at least a portion of the recovered unreacted anhydride to a reaction zone for reaction with an alkenyl material.