At the time Le Bras and one of the present authors pointed out the possibility of combining unsaturated compounds with rubber in the form of latex or by an ordinary mixing operation, reference was made to an interesting work, published by Bacon and Farmer in 1938, in which these authors described the addition of maleic anhydride to rubber in solution. At the same time it was emphasized, and this point was taken up in more detail in a later publication of Le Bras, that there are essential differences between the products obtained by the two methods, both with respect to their appearance and their properties, according to whether the technique employed is emulsion or mixing, or is that described by Bacon and Farmer. It should be borne in mind that the most distinguishing feature is that the first type of product is always to a great extent insoluble in solvents of rubber, whereas the second type of product may still be soluble even when 10–15 per cent, for example, of maleic anhydride has become fixed by the rubber. However, in spite of the products differing in certain attributes, the reactions involved in these two different experimental techniques should be fairly similar. Accordingly, in attempting to increase the action of these reagents on rubber, both in the form of latex and in solid admixture, and to throw further light on the mechanism of the reaction, the present authors have studied the effect of the reagents under the operating conditions employed by Bacon and Farmer, i.e., in solution. In what follows, various observations are reported which may contribute to a better understanding of certain peculiarities of this last type of reaction and to its wider application.