The microstructure of Ni-W alloys electrodeposited at 1000 A/m(2) from ammoniacal citrate solutions was discussed in terms of their codeposition behavior. When the W content in the alloys was increased, the partial current efficiency of W first increased toward a maximum value and then decreased. This showed the existence of two characteristic alloy compositions, i.e., a W content at a maximum partial current efficiency of W and an upper limiting content of W. The deposition behavior of Ni-W alloys was explained by the multistep mechanism of the reduction of the electrochemically reduced W(IV) oxide to W by atomic hydrogen adsorbed using the unpaired 3d electrons of the freshly deposited Ni. X-ray diffraction (XRD) analysis revealed that alloys with a low W content consisted of a Ni solid solution, and the diffraction peaks became broader with an increase in W content. Ni-W alloys with high W contents showed a smooth surface composed of fine grains. Transmission electron microscopy studies revealed that the major phase in Ni-W alloys was the Ni solid solution, and the XRD pattern of the Ni-53.9 mass % W alloy revealed a halo pattern characteristic of an amorphous structure. This alloy consisted of crystal grains smaller than 10 nm in diameter.