This experimental study addresses the behavior of binary mixtures of argon and sulfur hexafluoride in a rod-plane electrode configuration when subjected to impulse voltages. Positive-polarity, standard lightning impulse voltages (1.2-/spl mu/s/50-/spl mu/s) with different magnitudes were employed throughout the experiments. The percentage of SF/sub 6/ in Ar was varied from 1% to 90% at a total gas pressure of 50 kPa and 100 kPa. The breakdown voltage of each gas mixture was recorded at 50% breakdown probability. Similar to our theoretical as well as experimental studies under static electric fields it is observed that the breakdown strength of Ar has increased dramatically when mixed with a small percentage of SF/sub 6/. However, the breakdown voltages did increase monotonically but not dramatically for higher SF/sub 6/ percentages.