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The ac corona onset and breakdown voltages have been measured for rod-plane and rod-rod gaps in hydrogen, SF6, and hydrogen-SF6 mixtures for pressures between 1 and 5 bar, with gaps up to 14 cm, and voltages up to 250 kV rms. The addition to hydrogen of quantities of SF6 as low as 0.002% by volume resulted in a substantial increase in breakdown voltage of typically 70%. The corona onset voltage of the hydrogen was only increased by large additions of SF6 above 1%. With the mixtures for rod-rod gaps, breakdown values were obtained up to 70% higher than for SF6 alone, and there was no maximum in the breakdown voltage-pressure characteristic in contrast to the case for SF6. Very intense, stable glow currents of up to 20 mA were recorded. The characteristics and mechanisms of the various corona modes are discussed.