The initial permeability &mgr;iand loss factor tan&dgr; of amorphous Fe‐Co ribbons of low magnetostriction were measured at room temperature in the high frequency region from 1 kHz to 1000 kHz; the variation of &mgr;iat 10 kHz with T was also measured from room T to 300 °C. The initial permeability of as‐prepared Fe5Co73Si8B14ribbons at room T was observed to be about 600 from 1 kHz up to 500 kHz, and then to fall off. The loss factor tan&dgr; was rather small at low frequencies and increased rapidly from about 100 kHz and above. At 500 kHz, its value went up to 0.32. An irreversible T dependence of &mgr;ifrom room T to 300 °C and a disaccommodation‐like behavior of &mgr;iwith time were observed.