The production of PH3 from red phosphorus and the atomic hydrogen formed in the catalytic decomposition of H-2 on heated W surfaces was confirmed. The absolute density of the PH3 could be as high as 10(13) cm(-3); the density was proportional to H-atom density in the absence of red phosphorus, although three H atoms must be involved in the production of one PH3 molecule, but showed minor dependence on the amount of red phosphorus. These results suggest that the rate-determining step for the production of PH3 is that which produces H atoms. (C) 2011 Elsevier B. V. All rights reserved.