Among these techniques, frequency response has the unique advantage of being highly sensitive to the rate-limiting mass transfer mechanism. Gas molecules encounter a series of mass transfer resistances, including fluid phase film resistances at the surface of the adsorbent, diffusion through macropores, possible restrictions at the mouth of the micropores, and diffusion through the micropores. Any of these steps may be rate limiting, but frequency response can directly identify the limiting mechanism based on the shape of the frequency response curve. In particular, frequency response involves the periodic perturbation of some thermodynamic variable of a system at adsorption equilibrium while monitoring the response in another system variable. The ratio of the amplitude of the response variable to the amplitude of the perturbation as a function of frequency reveals details about the rate and mechanism of mass transfer. Commonly, frequency response involves the perturbation of the volume of a batch system while measuring the pressure response, perturbing the inlet pressure in a flowthrough system while measuring the response of flow rate, or perturbing the inlet concentration of the adsorbate in a flowthrough system and measuring the response in the effluent concentration. Among these, flowthrough frequency response techniques can generally be treated as isothermal, but heat effects can also be accounted for and identified from the response curves. Concentration swing frequency response (CSFR) is a flowthrough frequency response technique in which the concentration of adsorbate fed into a small bed of adsorbent material is periodically perturbed, usually sinusoidally, and the response of the concentration at the bed effluent is monitored. Because this method inherently involves using a carrier gas, it provides an opportunity to examine the concentration dependence of diffusion.