Hydrogen fusion may require a mixture of liquefied or frozen D2 and T2. The equilibrium constant of the exchange reaction describes the composition of this fuel. We have calculated the equilibrium constant K/sub DT/ for the reaction D2 + T2 = 2DT in the 4.2 to 100 K temperature range. The results agree well with previous calculations at 25, 50, and 100 K. Calculations at temperatures below 25 K have not been published previously. In the 16.7 to 33.3 K temperature range, which includes the triple point, K/sub DT/ can be represented by K = 2.995 exp( -10.82/T). The values of the analogous equilibrium constants for H2-D2 and H2-T2 are also given in the 4.2 to 50 K temperature range.