In the context of French nuclear waste vitrification, specific borosilicate glass waste forms are being developed to immobilize the large quantities (> 6 mol%) of molybdenum and phosphorus (2 mol%) oxides produced by reprocessing uranium-molybdenum spent fuel. The presence of these elements at high concentrations induces liquid-liquid phase separation during melting. In order to control the microstructure of the phases formed and to define the vitrification operating conditions, it is crucial to determine the phase separation temperature. This study is based on the melt rheological behavior during the phase separation step, and focuses on determining the phase separation temperature by measuring the transition of the liquid to non-Newtonian behavior or a deviation from the Vogel-Fulcher-Tammann (VFT) law. The transformation mechanisms in glass containing 6 mol% molybdenum oxide are first described as a function of temperature using various microstructural characterization techniques at microscopic (scanning electron microscopy, Raman spectroscopy) and submicrometer scale (transmission electron microscopy). These results are correlated with rheological measurements to determine the phase separation temperatures of molten borosilicates containing 5-6 mol% molybdenum oxide. The immiscibility temperature of liquid-liquid phase separation can be determined by the change in the liquid rheological behavior. The transition of the liquid to non-Newtonian behavior or a deviation from the VFT law are indicators of liquid-liquid phase separation observable at submicrometer scale.