The following calculations were made in connection with recent work on the kinetics of the attack on platinum by iodine vapour at low pressures. A platinum filament was heated in a glass bulb to which iodine was admitted at a pressure of 0·027 mm. of mercury from a reservoir of iodine kept at 0°C. The tap to the reservoir was opened for a period of two minutes, which was evidently sufficient to allow the iodine to saturate the walls of the bulb, since on closing the tap the pressure remained constant. On heating the filament, reaction ensued, causing a steady fall in pressure. This does not however truly represent the “clean up” due to the reaction, as the gas initially adsorbed on the walls of the bulb is continuously desorbed as the pressure falls. To obtain the true pressure decrease due to the reaction, a quantity representing the gas desorbed must be subtracted from the pressure reading at any moment.