Abstract: A method and apparatus are provided for adsorbing water vapor from a mixture thereof with a second gas to reduce the water vapor concentration in the mixture to below a permissible maximum concentration. The advance of the moisture front in a sorbent bed during either adsorption or regeneration or both is detected by determining the change in the moisture content of the sorbent as a function of the capacitance of a capacitor in which the sorbent is the dielectric. During adsorption, capacitance must be observed by way of a capacitor at least at a point in the bed sufficiently far from the end of the bed to prevent effluent having a limiting maximum moisture content from leaving the bed. When the capacitance at that point has reached a predetermined value corresponding to a selected moisture content on the sorbent, the adsorption or regeneration cycle is discontinued.