Intensity variations observed in light (λsimilar, equals0·95 μm) transmitted normally by thin magnetic films of Fe, Ni-Fe (81-19) and Co with uniaxial anisotropy when switched along their hard direction, are attributed directly to magnetic birefringence. The intensity changes are of the order of one part in 10³.Theoretical considerations enable the form of the intensity variations to be predicted, and show that measurement of their magnitude for a particular optical configuration leads to the magnitude of the complex magneto-optic parameter.