The exposure of a polycrystalline tungsten surface to atomic nitrogen or bombardment by N2+ is shown to enhance chemisorption. Ion bombardment causes the saturation coverage to be increased by about 50 % over that found after exposure to the molecular gas. A somewhat greater saturation coverage is obtained by exposure to neutral atomic nitrogen. The additional gas found in these experiments is desorbed at lower temperatures than nitrogen chemisorbed in β sites. These results are interpreted on the basis of chemisorption at endothermic adsorption sites. © 1971.