The synthesis of uranium diboride (UB2) from uranium dioxide (UO2) has been carried out for the first time after a coordinated experimental and theoretical investigation. The reliable conversion of UO2 to UB2 is of importance when considering commercially relevant products (e.g. as an advanced technology fuel - ATF), avoiding the use of uranium metal as a reactant. UO2 was reduced and borated in-situ through careful combination with boron carbide (B4C) and graphite (carbo/borothermic reduction). The reaction is observed to only be favourable at low partial pressures of CO, here made possible through use of a vacuum furnace at temperatures up to 1800 ∘C. At higher partial pressures of CO, the product of the reaction is UB4. For phase pure UB2, excess B4C is required due to the formation of volatile boron oxides that are released from the reaction mixture as is observed when synthesising other borides through similar routes.