The equilibrium constant for the gas phase isotopic exchange reaction D2+ 2 HCl ⇋ H2+ 2 DCl is evaluated by means of isotopic partition function ratios. A number of methods of calculation are explored. The preferred method here is the variational calculation of rotational-vibrational energy levels corresponding to appropriate potential curves, followed by the calculation of the partition functions by direct summation. In order to obtain agreement with experiment, it is necessary to introduce corrections to the Born-Oppenheimer approximation.