Mathematical models of perfectly mixed dual permeators with permeate recycles, operating with zero reject flow rate and zero permeate pressures (gamma＇ = gamma " = 0), have been developed. The equations for determination of the permeate compositions, the membrane area requirements and the separation factors have been derived, taking into consideration two different cases, one permeate recycle and two simultaneous permeate recycle streams. The simulation results have been presented for the separation of a CO2/H-2 mixture in a permeator enclosing both silicone rubber and cellulose acetate membranes. As silicone rubber membrane is more permeable to CO2 than to H-2, while the opposite is true for a cellulose acetate membrane.