Sealed chambers containing saturated solutions of certain salts are often used as simple methods of providing atmospheres of known humidities so that materials and components enclosed in the chambers may be brought to a standard condition of moisture content. A series of experiments at constant rates of absorption using ammonium nitrate as the standard salt has shown that the method cannot be relied upon to produce atmospheres of known humidities except for loads which absorb moisture at a negligible rate. For this salt the standard relative humidity is 65% when operating at 20° C but a rate of moisture absorption as low as 4 mg h-1 is sufficient to depress the humidity by about 2%. The depression of humidity is somewhat reduced when the atmosphere is stirred.