Scanning and transmission electron microscopy (TEM) were used to characterize the microstructure of electrodeposited multilayer Cu-Co alloys. Both columnar alloys (where the Co is localized to the columns) and three-dimensional arrays (where the Co is present in regions less than 1 mum diam) were studied. The Latter type was produced by successively electrodepositing a columnar alloy layer between Cu spacer layers, resulting in a semiordered, three-dimensional network of Co-rich regions in a principally Cu matrix. The composition gradient between the Co-rich regions and the Cu matrix was verified by electron energy loss spectroscopy and energy-dispersive spectroscopy using TEM. It was found that the size of the Co-rich regions was affected more by the overall alloy layer composition than by the deposition parameters. Under appropriate conditions, it was possible to reduce the individual alloy layer thickness to 25 run while maintaining chemical segregation between the Co-rich areas. Annealing at 250 degreesC and higher caused the Co-rich regions to merge together.