The characteristics of electrolytes will be described which prevent composition gradients from developing as the result of differences in Ni++ and Fe++ diffusion, during the electrolytic formation of Ni-Fe films. This is accomplished by using an electrolyte additive which results in a solution which can be operated at a Ni-Fe ratio that is the same as desired in the film, making both the Ni++ and Fe++ deposition diffusion controlled. Using a specific electrolyte with these characteristics, the influence of the substrate on the film composition, structure, and resultant properties has been demonstrated by changing the composition gradients induced by electrolyte diffusion effects. The metallurgical structure (grain size, porosity, crystal texture, composition) and the magnetic properties ( , magnetostriction) of films deposited on Au and Cu were examined. Using the Au substrate, films were made at constant deposition conditions from 100 to 4000 Å thick. The average Fe content of the films changed from 28 to 18 percent over this thickness range. Films deposited under the same conditions onto Cu substrates had a lower average Fe content; e.g.,13 percent at 600 Å thick. Film compositions as a function of thickness are described by the sum of two exponential terms; one term takes into account electrolyte diffusion effects on composition and the other term takes into account the electrode effect on composition. Previously reported data are analyzed in the same manner and composition gradients as a function of film thickness are confirmed to be larger than in the work reported here. Low values of coercive force and dispersion were obtained only from films thicker than about 300 Å deposited on Au. These low coercive forces and dispersion are shown to be due to the isotropic crystal structure, fine grain size, and relatively mild composition fluctuations in the Ni-Fe promoted by the characteristics of the Au substrate in the electrolyte. On Cu the films were magnetically isotropic and high in Hcbelow thicknesses of about 4000 Å, due to the highly porous film structure and large grain size induced by the Cu substrate. The deposition potentials, monitored- during the formation of the film, reflected the composition of the alloy depositing and provides a technique for choosing the optimum substrate and electrolyte.