In the nitrogen‐niobium system, sorption at high constant temperature and low pressure follows approximately the parabolic rate law where is the sticking probability at the nitrogen concentration in the metal, the initial sticking probability, and the saturation concentration. This rate law can be explained by a reversible reaction mechanism: Nitrogen is adsorbed atomically on the essentially clean metal surface at a rate which is proportional to pressure but independent of temperature; it then transfers rapidly into the bulk and distributes itself uniformly by fast diffusion. In the reverse reaction nitrogen recombines on the surface at a rate which is proportional to the square of the nitrogen atom concentration in the bulk. The observed degassing rates at both constant and variable pressure can be explained by the same mechanism.