The sensitivity of magneto-optical spectra to alloy chemical ordering is established. This is demonstrated for a new ordered CosPt phase, which was synthesized in thin film form using molecular beam epitaxial growth. This compound consists of alternating planes of Co and CoPt atoms with a hexagonal ABAB... layer sequence. The chemical ordering is accompanied by a substantial electronic structure change as reflected in a new peak in magneto-optical spectra at ~ 3.2 eV photon energy PACS numbers: 78.20.Ls, 61.66.Dk, 81.30.Bx Magneto-optical effects have attracted a lot of attention recently in research of magnetic thin films and multilayers. Strong resonance effects due to reduced optical constants [1,2], quantum confinement effects in ultrathin Fe layers [3], oscillations of the Kerr rotation with magnetic layer thickness due to quantum well states [4,5] and strong correlations between the magneto-optical Kerr effect (MOKE), and magnetic anisotropies [6] are among the most exciting recent discoveries in this field. While magnetostructural correlations have been known for a long time [7], the possibility of chemical ordering-related effects in the magneto-optical spectra has been suggested only recently [8,9]. With the advent of improved bandstructure-based magneto-optical calculations [10], it is possible to understand magneto-optical transitions from first principles and predict the effects of material structure upon Kerr rotation. In particular, the effect of chemical ordering on magneto-optical properties provides an important test of current theory. Co-Pt alloys provide an attractive candidate material for such studies since the Co-Pt bulk phase diagram reveals a continuous series of solid solutions with ferromagnetic order up to 90 at. % Pt at room temperature and the existence of chemically ordered CoPt and CoPt3 phases [11]. No chemically ordered compound is known for the composition CoaPt [11], although an LI2 cubic phase has been predicted in a recent calculation by Sanchez et at [12]. In this Letter we report the observation of a chemically ordered Co3Pt structure in films grown by molecular beam epitaxy (MBE). More importantly, we report the first unambiguous correlation between chemical ordering and magneto-optical Kerr rotation. 1000 A thick Co77Pt23 alloy films were deposited by MBE at various temperatures onto 10 A Pt buffer layers which were grown at 875 K on sapphire (0001) substrates. This results in high quality (111) [or (0001)] oriented alloy films [13]. The deposition chamber was maintained at a pressure < 4 x 10~1 0 mbar throughout growth. Electron gun sources were used for both Co and Pt, with deposition rates in the range 0.05-0.2 A/s. All compositions were checked with x-ray fluorescence. The spectral dependences of the polar Kerr effect were measured in saturating fields of ±20 kOe in the photon energy range 0.8-5.3 eV [14]. The saturation magnetization was determined using vibrating sample magnetometry. All magnetic and magneto-optic measurements were made at room temperature.