In the framework of modeling PSA processes, column dynamics for the adsorption of mixtures of helium and carbon dioxide on activated carbon has been investigated under conditions where both flow rate and column pressure vary due to adsorption. A model has been developed based on the usual approximations of local equilibrium and isothermal conditions but allowing for changes in flow rate as well as column pressure. Experimental breakthrough curves were measured up to 27 kPa, corresponding to 20 mol% carbon dioxide. Good agreement between model and experimental curves was obtained up to 6 kPa and fair agreement up to 27 kPa carbon dioxide partial pressure. Adsorption isotherm data, required in the model calculations, have been obtained from these experimental step responses. These results are in good agreement with experimental equilibrium adsorption data obtained from both a static volumetric method and a gravimetric method.