ABSTRACTWe have studied 100nm thick electron beam evaporated Co1-xNix alloy films in the composition range 0 ≤ x ≥ 1 using magneto-optical spectroscopy and magnetic anisotropy measurements. For films with x ≈ 0.2 we have also investigated the dependence of these quantities on the growth temperature, which was varied in the range 27 ≤ TG ≥ 408°C. Both as function of the Ni content x and the growth temperature TG we observe the anticipated hcp → fcc phase transition, e.g. by monitoring the magneto-crystalline anisotropy constant Ku, 1, which changes continuously from values of ≈ 0.3MJ/m3 for Co rich hcp alloys (0 ≤ x ≥ 0.25) to ≈ -0.05MJ/m3 for Ni rich fcc alloy films, in good agreement with bulk literature data. The new and most striking result, however, is observed in the polar magneto-optical Kerr and ellipticity spectra, which were measured in the photon energy range 0.8 ≤ hv ≥ 5.5eV. Changes by up to about 40% in Kerr rotation for films of constant composition and magnetization are observed when the structure changes from hcp → fcc. This demonstrates the sensitivity of magneto-optical effects to structural changes, making Kerr spectroscopy a useful electronic and physical structure probe.