This paper describes the effect of temperature on the mass transfer of niobium alloys in lithium as determined in thermal convection loop tests to 1300°C. All loops were operated for 3000 h in an ultrahigh vacuum environment; A consistent finding in these tests was the preferential movement of zirconium and nitrogen between heated and cooled regions, which in 1200°C tests accounted for most of the observed weight changes. At 1300°C, transport of niobium was also reflected in the weight changes, and crystalline deposits of Nb(C,N) were found on hot-leg specimens. Examination of Nb-1% Zr specimens using an electron microprobe indicated the hot-leg surfaces to be depleted of zirconium to depths of 0.002 and 0.007 in. in 1200 and 1300°C tests, respectively. The results of calculations of the diffusivity of zirconium in niobium based on these data agree well with values predicted from extrapolation of higher temperature data.