Measurements of the field modulated microwave absorption of YBa 2 Cu 3 O 7 , thin films as a function of temperature and magnetic field are presented. The Fourier spectrum of the reflected microwave power below T c reveals a peak at the fundamental frequency of the modulation field and its even harmonics. The amplitude of the even harmonics decreases rapidly when the dc field superimposed on the modulation field is increased, then shows a maximum and vanishes above a characteristic field B * . We show that this maximum is related to the full penetration of the modulation field and that the B * (T) curve can be viewed as an irreversibility line which is consistent with that obtained from irreversible magnetization measurements.