The instability of unprotected HgI2x‐ray detectors in vacuum, contrasted with their apparent stability in air is examined. The degradation is treated as a result of noncongruent sublimation of Hg and I2from the detector surface, resulting in a buildup of free Hg atoms. The calculations indicate that the observed degradation of these devices in vacuum is to be expected, and that similar degradation in air will occur at rates 103–104times more slowly.