The paper describes measurements, at a frequency of 1 000 c/s, of the capacitance changes in air capacitors due to moisture in the atmosphere. It is shown that these changes may be considerably in excess of those due to the alterations in the permittivity of the air. Over the range of relative humidity 30–65% the total change in capacitance can be about 3 parts in 10 000 at room temperatures. The full advantage of the otherwise high stability of a precision air capacitor is lost when this effect is ignored.