An instrument for the purpose of detecting small changes in air dielectric capacitance is described. It consists of an RC bridge circuit with the associated power supply and detector. The bridge has a 26 VAC input and sine wave output which is compared with a square wave in the detector. The dc output from the detector is a measure of bridge unbalance in millivolts, with a minimum sensitivity of 10́℗đ pico-farads at 200 pico-farads. The bridge and detector assembly were designed to monitor the rate of diffusion of gases and were used successfully for this purpose. A small change in gas mixture composition changes the gas-dielectric constant; therefore, the capacitance value of a small air-type capacitor inserted into the gas mixture changes proportionally, and is recorded as a function of time.