The effect of alpha-Al2O3 particles on the electrochemical codeposition behaviour of Co-Ni alloys from sulfamate electrolyte was investigated by codeposition experiments, zeta potential and electrochemical methods. The results indicate that the presence of Al2O3 particles in Co-Ni sulfamate electrolytes contributes to the preferential codeposition of Co. Moreover, increasing Co2+ concentration also can enhance the codeposition of Al2O3 particles in Co-Ni alloy matrix. Zeta potential measurements confirm that Co2+ has a stronger tendency to adsorb on Al2O3 particles surface than Ni2+ does in sulfamate electrolytes. Steady-state polarization and electrochemical impedance spectroscopy (EIS) show that the effect of alpha-Al2O3 particles on electrochemical codeposition behaviour of Co-Ni alloy in different Co2+/Ni2+ ratio electrolytes (1:5 and 5:1) is quite contrary. In nickel-rich electrolytes (Co (2+)/Ni2+ = 1:5), the Al2O3 particles cause a negative shift in reduction potential and an increase of charge transfer resistance. However, in cobalt-rich electrolytes (Co2+/Ni2+ = 5:1), Al2O3 particles lead to a positive shift of reduction potential and reduce the charge transfer resistance. Moreover, in EIS, besides one capacitive loop at high frequency and one inductive loop at low frequency, the introduction of Al2O3 particles leads to a further inductive loop at low frequency. According to simulated results of EIS, an electrochemical codeposition mechanism of Al2O3 particles with Co-Ni alloys is presented and in good agreement with the experimental results. (C) 2004 Elsevier B.V. All rights reserved.