The　influence　of　the　wavefront　duration　on　impulse　breakdown　voltages　is　reported　for　three　nonuniform　gaps:　a　100-mm　point-plane　gap　and　a　100-mm　cylindrical　rod-plane　gap,　both　in　air,　and　a　35-mm　point-plane　gap　in　SF//6.　For　comparison　purposes,　the　impulse-breakdown-voltage-wavefront　relationship　was　also　measured　for　a　10-kV　indoor　post　insulator.　The　wavefronts　were　0.　6,　1.　2,　18,　34,　80,　and　190　mu　s　while　the　wave　tail　duration　was　about　1800　mu　s　in　all　cases.　Experiments　showed　that　while　the　wavefront　had　an　insignificant　effect　on　positive　breakdown　voltages,　it　drastically　influenced　the　negative　breakdown　voltages.　Negative　breakdown　voltages　under　long-wavefront　impulses　may　be　as　high　as　1.　6　times　those　under　steep-fronted　impulses.　The　highest　impulse　ratio　measured　was　2.　In　contrast　to　the　air　gaps,　the　wavefront　had　a　very　pronounced　effect　on　positive　breakdown　voltages　for　the　SF//6　gap.　The　impulse　ratio　under　positive　polarity　varied　from　0.　62　to　1.　35　within　the　wavefront　range　studied.