Theη toG phase transformation in electrodeposited iron-zinc alloy coatings is a rapid diffusional phase transformation occurring in the nanometer-scale microstructure of the coatings. Iron-supersaturated zinc, orη phase, undergoes a decomposition process to an iron-deficient Γ-like, orG, phase at temperatures near 150 it°C. The phase transformation is preceded by a polygonization process, driven by strain energy and/or supersaturation in the material. TheG phase nucleates on boundaries within theη phase and grows into 30- to 80-nm equiaxed grains. The activation energy for the transformation varies with iron content, which may indicate that solute effects influence boundary diffusion of zinc.