1. Hydrogenation of natural and synthetic cis-1,4-polyisoprene has been accomplished. Polymers of lower molecular weights are more easily hydrogenated. 2. Hydrogenation of cis-1,4-polyisoprene at temperatures of 80–140° are not followed by cis-trans isomerization and apparently is not accompanied by cyclization. 3. The polymers have higher glass temperatures as the degree of hydrogenation is increased.