Syndiotactic polystyrenes (sPS) with different molecular weights were hydrogenated over the Ni/SiO2 and Pd-BaSO4 catalysts. Although the Ni catalyst yielded a completely hydrogenated sPS with a lower molecular weight, the hydrogenation of sPS with a high molecular weight was incomplete. On the other hand, the Pd catalyst was capable of hydrogenating sPS with a high molecular weight. However, the hydrogenated sPS (HsPS) was found to contained a small quantity of isolated styrene units. From a detailed analysis of the HsPS by differential scanning calorimetry (d.s.c.) and X-ray, it was revealed that this HsPS has a potential as a crystalline material with an excellent heat-resistance.