A reaction model has been developed to account for the mechanism of Zn-Ni alloys deposition from acidic sulfate electrolytes. The heterogeneous reactions imply the presence of four adsorbates : (i) ZnNiaddivided by which catalyzes nickel deposition, (ii) the intermediate H-ad involved in hydrogen evolution which occurs at low polarizations on nickel-rich deposits, (iii) the intermediate adion Zn-ad(+) acting as a catalyst for the deposition of the zinc-rich delta-phase which prevails at high polarizations, and (iv) the anionic species Zn/NiSO4,ad2- which inhibits the formation of the delta-phase. In the transition region of intermediate polarizations, an increase in the interfacial pH facilitates the chemical incorporation of S in deposits, which is catalyzed by the anionic adsorbate. The model validity has been confirmed by comparison with experimental polarization curves, electrode impedance measurements, and analyses of the deposit structures and compositions.