Magneto-optical spectroscopy has been used for the first time to detect phase separation in a magnetic system. The polar Kerr rotation of amorphous UxAs1-x films was measured in the photon energy range from 0.65-3.5 eV at various fields well below the Curie temperature. At concentrations x≥55 at. % U, the Kerr spectrum is dominated by a broad, negative peak near 1.1 eV that we associate with an U5f→6d transition. For x<55 at. % U, a positive peak appears below 1 eV. The positive peak has much higher coercivity and remanence than the negative peak, indicating a phase separation. The two amorphous phases could possibly be metallic UAs and semiconducting UAs2, based on a percolation analysis of the composition dependence of the resistivity.