The electrodeposition of magnetic cobalt-aluminum alloys was investigated in the Lewis acidic aluminum chloride-1-methyl-3-ethylimidazolium chloride [60.0-40.0 mole percent (m/o)] molten salt containing electrogenerated Co(II) at 25 degrees C. Rotating disk electrode voltammetry indicated that it is possible to produce alloy deposits containing up to 62 atomic (a/o) aluminum at potentials positive of that for the bulk deposition of aluminum. The onset of the underpotential-driven aluminum codeposition process occurred at around 0.40 V vs. the Al/Al(III) couple in a 5.00 mmol liter Co(II) solution but decreased as the Co(II) concentration increased. The Go-AL alloy composition displayed an inverse dependence on the Co(II) concentration but tended to become independent of concentration as the potential was decreased to 0 V. A rotating ring-disk electrode voltammetry technique was developed to analyze the composition and structure of the Co-Al alloy deposits. This technique takes advantage of the fact that the mass-transport-limited reduction of cobalt(II) occurs at potentials considerably more positive than that at which aluminum codeposition occurs. Scanning electron microscopy and energy dispersive x-ray analysis of bulk electrodeposits revealed that deposit morphology depends strongly upon aluminum content/deposition potential; deposits produced at 0.40 V from 50.0 mmol liter(-1) Co(II) solutions consisted of 10 to 20 mu m diam multifaceted nodules of pure hcp cobalt, whereas those obtained at 0.20 V were dense and fine grained, containing about 4 a/o Al. Deposits produced at 0 V had the visual appearance of a loosely adherent black powder. X-ray diffraction measurements revealed a lattice expansion and a decrease in grain size as the hcp cobalt was alloyed with increasing amounts of aluminum.