According to the principles of simultaneous equilibrium and mass equilibrium, a series of thermodynamic equilibrium equations in Ni（II）-Co（II）-C2O4^2--NH3-NH4^＋-H2O system at ambient temperature were deduced. The diagrams of logarithm ion concentrations versus pH values at different solution compositions were drawn. The results show that Ni^2＋ and Co^2＋ can completely precipitate at pH less than 5.0 and the predefined Ni/Co ratios can be well kept in the precursor. The precursor morphology is granular aggregation. However, rod aggregation precursor is obtained in the pH range of 5.0-8.0, and fibre-shape precursor is got at pH value higher than 8.0. The Ni/Co ratios in the above two precursors are not reproduced as that in the feed due to the formhtion of multi-coordinated Ni（NH3）n^2＋ and Co（NH3）n^2＋ （n=1-6）. Modification of precipitation medium is favorable for the precursors to keep the predefined Ni/Co ratios of the feed in the pH range of 2.0-8.6. Meanwhile, the precursors with fibrous morphology can be obtained.
A novel precursor of nickel-cobalt alloy powders with an appropriate Ni to Co molar ratio was prepared under selectively synthetic conditions. The composition and morphology of the precursor were characterized by X-ray diffractometry （XRD）, scanning electron microscopy （SEM）, Fourier transform infrared spectrometry （FT-IR） and energy dispersive spectrometry （EDS）. The effects of pH value, reaction temperature, metal ion concentrations and surfactant on the morphology and the dispersion of precursor were investigated. The results show that the morphology of precursor depends on ammonia content in the precursor. A fibriform precursor is a complicated ammonia-containing nickel-cobalt oxalate. The uniform shape-controlled fibrous precursor is obtained under the following optimum conditions： ammonia as complex agent as well as pH adjustor, oxalate as coprecipitator, 50-65 °C of reaction temperature, 0.5-0.8 mol/L of total concentration of Ni2＋ and Co2＋, PVP as dispersant, and pH 8.0-8.4.
