Previous work by the authors indicated that the penetration depth of krypton ions into the molybdenum cathode of a glow discharge tube operated at 150 V was the order of 104Å after a 24‐h bombardment (C. Y. Bartholomew and A. R. LaPadula, J. Appl. Phys.31, 445 (1960)]. Additional results indicate that this penetration depth increases with time of operation of the discharge. A diffusion‐like process appears to be dominant in the initial period of bombardment, but later the removal of surface layers by sputtering becomes dominant. The penetration reaches a steady‐state condition after a sufficient period of bombardment.