Alloys of the II-VI semiconductor CdTe with Mg or Zn have tunable bandgaps in the range modelled to be optimal for integration as a top cell in a tandem photovoltaic device with a Si bottom cell. Leveraging the low-cost production techniques developed for CdTe, this pairing has the potential to produce low-cost and highly efficient tandem PV cells. However, the CdCl2 post-deposition bulk passivation process widely used for CdTe reduces the concentration of the ternary elements in the absorber, reducing the bandgap from the modelled values, and results in poorer activation than the same process applied to pure CdTe material. We study the mitigation of this loss during CdCl2 treatment by the development of an overpressure of MgCl2 and ZnCl2, thought to be products of the loss reaction, for Cd1-xMgxTe and Cd1-xZnxTe, respectively. Little effect is seen until a vapor pressure far in excess of CdCl2 is achieved, reducing the loss - as measured by transmittance - by half.