AbstractIsoprene–styrene diblock copolymers of various molecular weights were prepared by anionic polymerization with n‐BuLi in cyclohexane. Hydrogenation of the unsaturated backbone of the block copolymers was performed with diimide generated in situ by the thermolysis of p‐toluenesulfonylhydrazide (TSH). The reaction was carried out in xylene at 135°C. A molar ratio of TSH to double bonds equal to 4:1 was found to be the optimum ratio, which provided the highest percentage of hydrogenation. The percentage of hydrogenation analyzed by 1H‐NMR, IR spectroscopy, and iodine value (the Wijs method) were compared. Evidence from 1H‐NMR also revealed a change in the ratio of the cis–trans configuration after hydrogenation. The thermal stability of the hydrogenated products was improved as shown by the results from thermogravimetric analysis. From differential scanning calorimetry measurements, the glass‐transition temperatures of the hydrogenated products were found to increase about 10–20°C above those of the original block copolymers. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 76–82, 2003