In the synthesis of mixed nitrides of uranium and plutonium, the carbon behavior is determined not only by its excessive content against stoichiometry, but also the active influence of hydrogen and nitrogen as components of the process gas. It is important to take into account the influence of binder thermolysis products. The results of possible reactions of the thermodynamic analysis during the synthesis indicate a specific phase behavior UC1,91 in the process, and raises further questions on the mechanism of excess carbon removal at synthesis temperatures.