A systematic study of the ferromagnetic resonance (FMR) of the metastable (formula presented) granular alloy ribbons prepared by melt-spinning was made as a function of magnetic field angle relative to the normal to the plane of the ribbon, and annealing temperature. This study allowed changes in the Co grains to be followed. The measurements were performed at room temperature, for cobalt concentrations of (formula presented) 5, 10, and 15, with samples both as-cast and annealed for 1 h at temperatures between 400 and 800 °C. In order to obtain the resonance fields and linewidths, the FMR spectra were fitted as a sum of absorption and dispersion functions. We observed that the resonance fields and linewidths vary in a systematic way with the angle of the magnetic field, with Co concentration, and with the annealing temperature. These results were interpreted using the Kittel equation for ferromagnetism, as well as simulations of the resonance field with the angle derived for spherical superparamagnetic grains. The analysis of the FMR spectra indicates grains whose mean diameters vary between 1.4±0.1 nm and 4.1±0.1 nm, depending on the cobalt concentration and annealing temperature. In addition, measurements of (formula presented) annealed samples show a reduction in their magnetization at certain temperatures. This reduction is probably induced by a change in the crystal structure of the cobalt grains, or by the dissolution of the smaller ferromagnetic grains in the Cu matrix. © 2001 The American Physical Society.