The method relates to the pyrolysis of a feedstock formed of oil containing chemically bound carbon in a reactor. The reactor is heated electrically, and the feedstock is heated over its vaporisation temperature and injected into the reactor; electric sparks are formed in the area where the vaporised feedstock is injected to ionize it before pyrolysis thereof, and the ionized vaporised feedstock is pyrolysed by the radiant energy of the reactor, whereby carbon black and a gas containing mainly hydrogen are produced. The plant comprises a reactor 1 with an inlet 2 and an outlet 3, electrical means 4 for heating the reactor 1, injector means 5 at the inlet 2, conduit means 6 for supplying the injector means 5 with the feedstock, means for heating the feedstock over its vaporisation temperature in the conduit means 6, and spark generating electrodes 8 arranged in the reactor 1 in the vicinity of the injector 5.