An analysis of the molecular structure of partially hydrogenated fractions of a monodisperse polybutadiene has been carried out in order to study the nature of the heterogeneous hydrogenation process. The polybutadiene was synthesized by polymerization of butadiene with t-butyl lithium in a non-polar medium. This reaction produces linear polibutadiene chains with about seven 1,2 addition units every 100 structural units. The catalytic hydrogenation of the polybutadiene fractions was done either in cyclohexane with Pd/CaCO3 catalyst or in heptane with Pd/BaSO4 catalyst. The reaction time was varied in order to obtain materials with different amount of unsaturations. The resulting polymers were characterized by gel permeation chromatography, differential scanning calorimetry, and infra-red spectroscopy. Solvent extraction using iz-hexane was also performed over thin films of several samples of hydrogenated polybutadienes. The resulting fractions of the non-extracted material, and of some of the extracted ones, were analysed using infra-red spectroscopy. The results show that the hydrogenation reaction generates a birnodal distribution oi polymer species for global conversions lower than approximately 89%. The catalytic reaction takes place in such a way that, in each adsorption step, approximately 89% of the unsaturated groups of each adsorpted molecule are hydrogenated. This process generates a mixture of polymers that are either highly hydrogenated poly(ethylene-l-butene-butadiene) copolymer or non-hydrogenated polybutadiene. The resulting blends are immiscible at all the studied global conversions (12-89%).