The paper considers the criteria of sensitivity and stability required for the determination of the dielectric constants of gases, and discusses the methods of measurement which have been used in relation to these criteria. The heterodyne method—which has been the most widely used—is considered to fail to meet the requirement of stability; other methods having the necessary stability are insufficiently sensitive. It is considered that the necessary stability and sensitivity can be achieved by the use of an a.c. bridge, a suitable design of which is discussed. Results are given to illustrate the performance of such a bridge, and it is shown that a change of capacitance of one part in six million is detectable, whilst the capacitance drift is normally less than one part in 106per hour. This drift is probably due to atmospheric changes. Results of the measurement of dielectric constants of several gases and of water vapour are given.