Quantities characterizing high temperature thermodynamic equilibria such as vapor pressures, enthalpies of formation and thermodynamic activity coefficients have traditionally been determined by classical techniques such as the Knudsen effusion method. Such classical techniques usually suffer from poor accuracy and from interference from other vapor species present with the vapor being measured. By comparing the classical methods with the rather infrequently used optical technique based on opacity measurements, we can demonstrate that this latter technique is largely unaffected by interfering vapor species and that it also possesses a spectroscopic multiplexing advantage that considerably enhances its accuracy over classical techniques. The attainment of these two advantages is illustrated with examples of atomic uranium opacity determinations in the pure uranium system and in the uranium nitride system. In both cases, all the opacities were obtained in the presence of interfering uranium oxide vapor and it was still possible to derive the desired thermodynamic data with excellent accuracy.