The adsorption phenomena of oxygen and nitrogen on a carbon molecular sieve were studied above the critical temperature of the adsorptives as a function of pressure in order to understand further the mechanism of air separation. The uptake of both gases studied was virtually linear at low equilibrium pressures, in agreement with Henry’s law, but deviation occurred at higher pressures. The adsorption kinetics were studied with different amounts of preadsorbed gas for changes in pressure of 11 kPa and partial pressure in helium of similar to 10 kPa. The gas adsorption kinetics obey a linear driving force mass transfer model. The ratios of the rate constants (kO(2)/kN(2)) for each pressure increment were 35-43 for pure gases and 21-27 for gas/helium mixtures, and these ratios clearly demonstrate the molecular sieving characteristics. The presence of water vapor is detrimental to the operation of carbon molecular sieves. The adsorption and desorption characteristics of water vapor with different amounts of preadsorbed water were studied for comparison with oxygen and nitrogen adsorption over the pressure range 0-1.8 kPa for pressure steps of 0.1 kPa. The results are discussed in terms of the mechanism of gas separation using carbon molecular sieves.