A method for measuring magnetic losses in amorphous alloys is described.It is based on the use of a lock‐in amplifier to determine the magnitude and phase of the fundamental component of magnetic field H when the induction B is forced to vary sinusoidally with time. The method is applicable to very small samples in the form of straight lengths or of toroids, and gives satisfactory results at flux levels approaching saturation if the flux wave form can be held sinusoidal. Data are presented on several amorphous alloy compositions as a function of peak induction and of frequency. Annealing effects have been investigated.