The properties of liquid Si–Ge binary systems under melting conditions deviate from those expected by the ideal alloy approximation. Particularly, a nonlinear dependence of the dielectric functions occurs with the reflectivity of liquid Si–Ge reaching a maximum at 50% Ge content, which is 10% higher than in pure Si or Ge. Using ab initio methodologies, we modeled liquefied Si–Ge alloys, unveiling very high coordination numbers and poor symmetry in the first coordination shell with respect to Si and Ge, related to different bonding properties. We simulated optical functions, quantitatively replicating the aforementioned reflectivity trend, and we highlighted a direct relationship between atomic structure and optical properties, indicating that unusual optics arises from Si–Ge higher local coordination characterized by low symmetry. We forecast further implications for the overall class of these alloys. These findings …