(a) providing a pressure swing adsorption system having a plurality of parallel adsorber vessels and a plurality of valves and gas manifolds adapted to introduce gas into each adsorber vessel and withdraw gas from each adsorber vessel in a cyclic series of sequential process steps including at least a feed step, a depressurization step, a regeneration step, and a repressurization step;

(b) operating each of the adsorber vessels in the cyclic series of sequential process steps including at least the feed step, the depressurization step, the regeneration step, and the repressurization step, wherein each of the plurality of valves is opened and closed at least once during the cyclic series of sequential process steps in a predetermined valve operating sequence;

(c) determining a value of an operating parameter that is a function of the mass of gas provided to a receiving adsorber vessel or withdrawn from an adsorber vessel during a selected process step, portion of a process step, or series of process steps;

(d) determining the deviation of the value of the operating parameter from a predetermined reference value; and

(e) using the magnitude and direction of the deviation of (d) to determine whether any valves are leaking, wherein (1) when the absolute value of the deviation of (d) is less than a predetermined minimum value, no valve is identified as leaking and (2) when the absolute value of the deviation of (d) is greater than the predetermined minimum value, at least one valve is identified as leaking.