Electrodeposition of the Fe-Ni alloys in the citrate bath was anomalous co-deposition, but normal co-deposition in the citrate-ammonia bath. The Fe and Ni elements were uniformly distributed in the Fe-Ni deposit, which were highly alloyed. These results cannot be simply explained by the hydroxide deposition model or the competitive adsorption of intermediate model, which proposed in literature. A poly-nuclear complex co-deposition mechanism of Fe-Ni alloy was proposed in the above two baths, which can explain better electro-deposition behavior of the Fe-Ni co-deposition and the experimental phenomena we observed. (c) The Author(s) 2018. Published by ECS.