A detailed Lorentz transmission electron microscopy study of melt-spun Au78Co22 and Au86Co14, and microstructural changes upon annealing, was conducted. The microstructure of the as-cast alloys consisted of small grains with fcc Co precipitates dispersed at the grain boundaries. The interior structures of these grains were Au/Co lamellar eutectic or very small Co precipitates dispersed in Au matrix, depending on the composition. Up to 10±1 at. % Co were dissolved in the Au matrix. This large amount of Co in the Au matrix reduced the mean free path, probably leading to low magnetoresistance values [MR(H=1.45 T)=2.4%–2.6%]. Annealing caused precipitation of Co out of the Au matrix and growth of some Co precipitates, which transformed to multidomain particles when exceeded 160–200 nm. Annealing at 673 K for 1 h caused a reduction of MR(H=1.45 T) to 0.7%–0.8%, mainly due to growth of Co precipitates. The magnetic and micromagnetic properties were correlated with the microstructural analyses using computer simulations. The Co lamellae and precipitates were single-domain particles and single-domain vortices, respectively.