Up to now catalytic copolymerization with alkali catalysts has hardly been studied. Only a few papers have been published which give quantitative data. Judging by the values of the copolymerization constants, monomers differ very considerably in their reactivities. Monomers in which the double bond of the vinyl group is conjugated with the multiple bond of a nitrile or a carboxyl group have the greatest activity. The copolymerization of diene hydrocarbons under the action of basic catalysts has not been investigated as yet. It is known that in free radical (emulsion) polymerization these hydrocarbons have relatively similar activities. It seemed interesting to establish the relative reactivity of the diene hydrocarbons, isoprene and 1,3-butadiene, which differ by only a methyl group, in alkali-catalyzed copolymerization. Labeled atoms were used for determining the composition of the copolymers. For this we synthesized isoprene containing radioactive carbon, C14 (from acetylene-C14, prepared from BaC14O3) by a modification of Favorskii˘'s method. Copolymerization and separate polymerizations of isoprene and 1,3-butadiene were carried out in hexane solution with butyllithium at 50°, with a total monomer concentration of 2 moles/liter and a catalyst concentration of 0.0025 mole/liter. Butyllithium was synthesized by the known method. The butyllithium solution was analyzed by the usual method with a double titration. Thoroughly purified and dried monomers and solvent were used for the polymerization. All operations of measuring out the products into the reaction ampules were carried out in a current of dry nitrogen and in a vacuum apparatus having a distributing comb (manifold). The reaction ampules consisted of two sections, separated by a wall and connected with a graduate tube. The solvent and monomers were placed in one section and the catalyst, a solution of butyllithium in hexane, in the other.