Diene rubbers are produced in the gas phase by first polymerizing the dienes or diene mixtures in the presence of a catalyst system, which is based on rare earth compounds, at temperatures of from 0 to 150.degree. C. and at pressures of from 1 mbar to 50 bar, so that a pourable diene rubber having a Mooney viscosity ML (1+4', 100.degree. C.) of 70 to 180 Mooney units is obtained, and subsequently subjecting the pourable diene rubber obtained to a chemical or thermal decomposition reaction until a Mooney viscosity of 10 to 70 Mooney units is obtained.