Poor surface finish and coarse dendrite structure are the major challenges in direct-current (dc) plating of nanosized Ni-Cu alloy coatings. This investigation was initiated to understand the effect of saccharin on the formation of nanosized Ni-Cu alloy coatings by sediment codeposition, and the role of saccharin in improving surface finish and suppressing coarse dendrite growth during sediment codeposition. It was found that only 0.5 g/L addition of saccharin could form dendrite-free nanocrystalline Ni-Cu alloy coatings with a mirror-finish surface. The Cu content in the Ni-Cu alloy coatings can be controlled to be as low as 10 wt % by changing the current density. The grain size in the coatings was determined by X-ray diffraction and electron microscopy analysis to be 15.7 nm on average. The amount of ordered L1(0)-type Ni-Cu nanophase was found to be insignificant in comparison with that in pulse plated coatings. Saccharin suppresses the reduction of Cu and acts as a leveling and grain size reduction agent in Ni-Cu alloy codeposition. From steady-state polarization and impedance analysis, it is believed that these saccharin effects are produced by the formation of Ni-Saccharin complexes adsorbed on the coating surface that suppress Ni-Cu dendrite growth. (C) 2008 The Electrochemical Society.