(1) Maleic anhydride is known to react (a) with conjugated compounds, e.g., butadiene, usually producing simple cyclic systems, and (b) with unsaturated olefinic substances like styrene and vinyl acetate, producing long-chain heteropolymers. It is now demonstrated that maleic anhydride, under the influence of a little benzoyl peroxide, will react in solution with rubber on heating, yielding a variety of tough, fibrous or resinous products. (2) It is shown that these derivatives are not produced by the action of maleic anhydride alone or of benzoyl peroxide alone, nor are they mixtures of rubber and maleic anhydride polymer. (3) As a result of the reaction, rubber has been found to undergo an increase in weight ranging from a few per cent up to about 120%, according to experimental conditions. The corresponding derivatives differ greatly from rubber in mechanical properties, and range from products which are tough, rubbery and non-tacky, to hard and brittle resins. (4) The effect of varying the quantity of benzoyl peroxide between 1% and 10%, and of using greater excess, is demonstrated. (5) The effect of varying the maleic anhydride/C5H8 ratio between 0.05 and 5 is demonstrated. (6) It is is shown that the nature of the solvent greatly influences the extent of reaction. The use of benzene, toluene, xylene, carbon terachloride, chloroform, decalin, and cyclohexane is discussed. (7) The influence of concentration and the occurrence of gelling have been investigated. (8) The derivatives are very different from rubber in their solubility characteristics.