Abstract: A process for treating a gaseous mixture, particularly air, by pressure swing adsorption (PSA), of the type in which three adsorbers (1 to 3) are used in each of which is carried out, for a given nominal production, a cycle comprising the following successive stages, the cycle being offset from one adsorber to the other by a third of the nominal duration (T) of the cycle:(a) a substantially isobaric adsorption phase at a high pressure (PM) of the cycle, by circulation of the mixture through the adsorber in a so-called co-current direction;(b) a desorption phase comprising a stage (b2) of pumping to a low pressure (P.sub.m) of the cycle less than atmospheric pressure, via a vacuum pump (7) of constant speed and continuous operation whose output is in the vicinity of atmospheric pressure; and(c) a repressurization phase of the adsorber to the high pressure of the cycle. During a reduction of the production flow rate, the mean intake pressure of the vacuum pump is raised for the duration of the cycle.