The breakdown strengths of dry air; sulphur-hexafluoride (SF6) and a mixture of dry air pills SF6 containing 1% of SF6 were studied. For this purpose, 50-Hz AC and lightning impulse breakdown voltages in uniform and non-uniform fields up to a pressure of 400 kPa were measured. The results show that the addition of 1% of SF6 to dry air increases considerably the breakdown voltage in a non-uniform field. By the aid of breakdown voltage versus pressure curves the relative breakdown strength of the mixture with respect to components was calculated both in 50-Hz AC and lightning impulse voltages. The analysis of Paschen curves reveals a slight improvement of breakdown strength in the case of mixture bur in non-uniform fields the maximum AC breakdown voltage of the mixture is slightly lower than that of pure SF6. However; for positive lightning impulses maximum breakdown voltage of the mixture was found to be greater than that of pure SF6 alone.