The hydrogenations of butadiene and isoprene prepolymers having carboxyl and cyano end groups were investigated by two methods : Heterogenous hydrogenations were carried out at 45 degrees C and 50 psi of hydrogen pressure with palladium on carbonate as catalyst. The best degree of saturation and carboxyl retention were 95 and 79% for butadiene prepolymers and 84 and 95% for an isoprene prepolymers, respectively. Diimide reduction was made in xylene at 130-140 degrees C and normal pressure with p-toluenesulfonylhydrazide (TSH) as the diimide source. A nearly 100% degree of saturation for butadiene prepolymers and 91% for isoprene prepolymers were achieved. The effects of reaction conditions on the degree of saturation and the carboxyl retention, calculation of molecular weight before and after hydrogenation based on H-1-NMR spectral data, and diimide reduction with TSH are discussed.