A study was made with the object of producing gas-impermeable sheaths from zirconium boride with 5% Mo additions and of testing such sheaths under industrial conditions. It is shown that the liquid phase which forms during sintering as a result of the reaction of molybdenum with zirconium boride gives rise to substantial shrinkage and helps to obtain gas-impermeable parts. Zirconium boride alloys with up to 10% Mo consist of a single phase.