Heating by means of saturated steam is used widely in the dairy industry in many processes. One application is autoclaving, a process not confined to the laboratory but used also for the preparation of sterilized milk(1). It is well recognized that the presence of air in autoclaves depresses the temperature of the heating medium when control of the process is based on measurement of pressure, but it does not appear to be generally appreciated, at least in the dairy industry, that air can depress the rate of heating of the load even when thermostatic devices are used to control the autoclave temperature. In the first place air will tend to accumulate in areas remote from the steam inlet in large vessels, with consequent reduction in partial steam pressure, and hence steam temperature, in these areas. Where the atmosphere temperature is maintained another effect occurs. This second effect is due to insulating layers of air which form on the cool surfaces of the load as the steam fraction of the mixed atmosphere condenses. A blanket of air thus formed is unable to disperse because of the general movement of the mixed atmosphere to the condensing surface.