Polybutadienes with different microstructure were epoxidized with in situ formed performic acid. The effects of the rubber microstructure, reaction time, molar ratio of the reagents (H2O2/ C=C/HCOOH) on the conversion of double bonds to oxirane groups was investigated. By means of H-1-and C-13-NMR- and FT-IR-spectroscopy it was demonstrated that a molar ratio of H2O2/ C=C/HCOOH equal to 3/2/1 ring opening side reactions do not occur until 70% of double bonds are converted. Under the same reaction conditions it was found that no gelation takes place and the reactivity of the cis/trans 1,4-butadiene units was significantly higher than the that of 1,2-butadiene units. Kinetic studies of the epoxidation rate show that, until a certain reaction time, depending of microstructrure of the rubber, the reaction follows a first-order time law with respect to hydrogen peroxide and acid concentration. From the temperature dependence of the epoxidation rate of polybutadiene with 47mol % 1,2 butadiene units an activation energy of 34 kJ/mol was determined. As expected the glass transition temperature increases linearly with the epoxiclation degrees for all the BR's under investigation.