An experiment is conducted to confirm the theoretical prediction that a rapidly generated lossy plasma can cause spectral breaking and frequency shift of a high-power microwave pulse. Spectral breaking is the transformation or breaking of a single dominant spectral peak associated with an incident pulse into two spectral peaks. The experiment is conducted by comparing the frequency spectrum of an incident pulse with the spectrum of the pulse transmitted through a self-induced air-breakdown environment. It is shown that as the ionization rate becomes too high, the spectrum of the transmitted pulse breaks up into two peaks: one has an upshifted centre frequency, and the other has a downshifted centre frequency. The results show that the amount of frequency upshift is correlated with the ionization rate, whereas the amount of frequency downshift is correlated with the energy losses from the pulse in the self-generated plasma. These experimental results agree with the theoretical prediction and a numerical simulation, which are also presented.