Oscillating flows is a novel separation method to aid in air revitalization in self-supporting environments that are important to NASA's advanced life support. This purely mechanical process can assist in the removal of carbon dioxide, fine particles, and other contaminants from the air. Separation can be achieved as oscillating flows markedly increase the mass transfer of a dilute species in a gas over that due to pure molecular diffusion. If two gaseous species are present in a carrier, the interaction between the time constants associated with the different diffusion coefficients, kinematic viscosity of the carrier, and the frequency of oscillation dictate the rate of mass transfer and amount of separation. The geometry of the system is also an important factor in the mass transfer and separation. Specifically, models have been developed to determine the mass transfer and separation in the annular space between two concentric cylinders. If the inner cylinder is slightly off center from the outer cylinder, the mass transfer decreases but the separation increases in relation to the oscillatory concentric case. The decrease in mass transfer is contrary to the result that would be expected in steady flows given by Taylor dispersion. A detailed explanation of the physics of oscillating flows and the unusual effects exhibited by different geometries will be presented. Published by Elsevier Ltd on behalf of COSPAR.