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A method is developed for computing the discharge threshold voltages in SF6, N2 as well as their mixtures above atmopheric pressure. In electronegative gases, and for both uniform and nonuniform field gaps of positive polarity, there is a critical pressure above which field emission from the cathode surface has a significant effect. The method of calculation is based on the condition necessary for the onset streamer to propagate. The effect of increased pressure over the critical value is that spark breakdown takes place within the discharge gap unpreceded by any corona. The computed values are in good agreement with those measured experimentally at pressures below and above the critical value for both uniform and nonuniform field gaps. The present calculations confirm those predicted experimentally, namely, there is no significant difference in the behavior of pure SF6 and mixtures up to 50 percent of N2. The fact that mixtures can show a higher breakdown voltage at the same total pressure than pure SF6 is also confirmed.