The oxidati n rates of Al in air in the presence of HgI2 were studied over a relative humidity range of 40 to 100. Measurements were made of time, temperature, and pressure at 5- to 10-min intervals until the oxidation slowed or completely stopped. The slopes of the linear portions of the rate curves were about the same at any particular relative hu idity, however, as the relative humidity decreased, the slopes of the rate curves also ecreased, At low relative humidities 55 and below the extent of oxidation was very small. At high relative h midities, the oxide was voluminous, sometimes growing in one continuous thick blue-white column. At low relative humidities, the oxide consisted of short fibers, somewhat powdery and pure white in color.