Two methods are described for the measurement of the double layer differential capacitance in media of low conductivity, ie under conditions in which the conventional bridge method fails: (a) measurement of the change of potential caused by coulostatic charging with a small excursion of potential; (b) determination, under coulostatic conditions, of the time constant of the circuit comprising the double layer capacitance in series with a large resistance. The first method is definitely the more convenient and slightly the more accurate. Application is made to 10 −3 to 10 −5 M sodium fluoride, and results agree with those calculated from the Gouy-Chapman theory for Grahame's capacitances of the compact double layer. The coulostatic method is compared with the bridge method and some potential applications are mentioned.