The synthesis process of Co-Ni alloy nanoparticles via electroless deposition in ethylene glycol (EG) was electrochemically investigated. Insufficient thermodynamic data for organic solvents prevented us from calculating the oxidation-reduction (redox) potentials of Co(II)/Co and Ni(II)/Ni redox pairs in organic solvents. In the present work, however, the redox potentials of Co(II)/Co and Ni(II)/Ni redox pairs in EG were determined by using voltammetry combined with quartz crystal microbalance. In the cyclic voltammograms, the deposition current of cobalt on cobalt substrate was much higher than that of nickel on nickel substrate, which resulted in decreasing particle size with increasing nickel amount. Moreover, it was experimentally clarified that a mixed potential was determined by each partial reaction. Specifically, the hydrazine oxidation reaction was dominant in this reaction system, which is a key to controlling the mixed potential.