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Surface discharges along oil-immersed solids used as insulators and supports in high-voltage pulsed-power equipment can lead to catastrophic system failures. To achieve reliable compact pulsed-power systems, it is important to quantify the electrical fields at which surface flashover, or other types of breakdown event, will occur for different dielectric materials. This paper reports the observed behavior of samples of polypropylene, lowdensity polyethylene, ultrahigh-molecular-weight polyethylene, Rexolite, and Torlon, which were subjected to impulse voltages of peak amplitude of 350 kV and a rise time of 1 μs. The cylindrical samples were located between pairs of electrodes immersed in insulating oil. Breakdown events were studied under both nonuniformand uniform-field conditions, with sample lengths being chosen so that the breakdown events occurred on the rising edge of the impulse. Ultrahigh-molecular-weight polyethylene showed the highest average breakdown field, which is 645 kV/cm, in uniform fields, and the corresponding breakdown field was reduced to ~400 kV/cm in the nonuniform fields. Weibull plots of the various sets of results are presented, providing comparative data for system designers for the appropriate choice of dielectric materials to act as insulators for high-voltage pulsed-power machines.