High-resolution and analytical transmission electron microscopy, X-ray diffraction, magnetic and magnetoresistance measurements were used to investigate nanostructures of melt-spun Au86Co14 and Au78Co22 alloys. The microstructure of the Au86Co14 alloy was composed of very small Co precipitates inside the Au grains with some larger Co precipitates (20-35 nm) dispersed at the grain boundaries, while the microstructure of the Au78Co22 alloy consisted of Au/Co lamellar eutectic grains with Co precipitates (50-70 nm) dispersed at the grain boundaries. A few grains had very small (4 nm) Co precipitates. Annealing at 773 K for 10 min caused Co depletion in the Au matrix from 5.4-10 at.% to 0.9-2.0 at.%. Annealing also caused transitions from superparamagnetic to ferromagnetic and from single to multiple domain magnetic structure of some of the small and some of the larger Co precipitates, respectively. The MR ratios (Deltarho/rho, in magnetic field of 14.5 kOe) of the as-cast Au86Co14 and Au78Co22 alloys were 2.5% and 2.6%, respectively. Annealing of the alloys at 673 K for 1 hr reduced Deltarho/rho to 0.9-1%. (C) 2003 Kluwer Academic Publishers.