An evaluation has been made of the net ionization frequencies of air inferred from breakdown measurements. This has been accomplished by comparison of such data with the corresponding values obtained from numerical solutions of the Boltzmann equation. Good agreement is found at high pressures, but the comparison deteriorates with decreasing pressure. This trend is attributed to the use of the effective field concept and the absence of nonuniform field corrections in the processing of the experimental data. Calculations are presented that show that breakdown fields calculated with data obtained from solutions of the Boltzmann equation and combined with an appropriate nonuniform field analysis are in good agreement with experimentally measured breakdown fields. It is concluded that the latter approach provides a more rational and accurate method for predicting high‐frequency breakdown than previously available methods.