The high-temperature phase diagram of the UO2–ThO2 system has been experimentally revisited in the present study for the first time since 1970, using a laser heating approach combined with fast pyrometry in a thermal arrest method. The melting/solidification temperature, which is of fundamental information for a reactor design was studied here. It was found that low addition of ThO2 to UO2 would result in a slight decrease of the solidification temperature. A minimum was found at 3098 K around a composition of 5 mol% ThO2. The solid/liquid transition temperature was then observed to increase again with increasing ThO2 fraction. The literature value of pure ThO2 (around 3630 K) was well reproduced here. Important experimental difficulties, stemming from the high temperatures reached during the measurements, as well as a complete investigation with electron microscopy, Raman spectroscopy and powder X-ray diffraction, are extensively discussed. These results show the importance of the high-temperature oxygen chemistry in this actinide oxide compound.