The viscosities of deuterium over the range 23 to −183 °C. have been measured. The viscosities of the two-component system deuterium-hydrogen have been measured over the whole concentration range at 22 °C. The results show that the deuterium molecule has the same diameter as the hydrogen molecule. The interest attached to the viscosity results for the two-component system are pointed out. The difference between the molecular volume of liquid deuterium oxide and that of liquid water is attributed by the authors to a difference in equilibrium between associated and non-associated molecules in the respective systems.