Helium desorption from carbon molecular sieve fibers was measured directly with a sensitive temperature-programmed desorption mass spectrometer with a supersonic molecular beam inlet apparatus. The effects of surface affinity and pore dimension on the peculiar strong interaction of the noble, "nonadsorbable" helium atoms with a carbon molecular sieve adsorbent, at high temperatures (100-400 K), are presented. It is shown that pore dimensions, as sensed by CO2 adsorption kinetics, are a key factor for the evident dramatic change in the interactions of helium with porous surfaces.