Selective H-atom line broadening was found to be present throughout the volume (14cm ID x 38cm length) of low pressure (0.03-0.4 Torr) H2O only RF plasmas generated in an RF capacitively coupled parallel-plate cell. An exhaustive study of the first four Balmer lines in water plasmas operated over a range of pressures and absorbed powers revealed significant trends. Of greatest interest was the finding that at low pressures (ca. < 0.08 Torr) a significant fraction (between 10% and 40%) of the atomic hydrogen was 'hot' with energies greater than 40 eV. The magnitude of the line broadening was found not only to be pressure dependent, but also to be a weak function of the energy absorbed by the system. The degree of broadening was virtually independent of the position studied within the RF capacitively coupled parallel-plate cell, similar to the finding for H-2/He and H-2/Ar plasmas in the same capacitively coupled cell, reported elsewhere. In contrast to the atomic hydrogen lines, no broadening was observed in oxygen species lines at low pressures. Also, in 'control' H-2/Xe plasmas run in the same cell at similar pressures and adsorbed power, little broadening of either atomic hydrogen, Xe or any other lines was observed. The values of the line broadenings observed in the low pressure water plasmas, and the fact that they were observed throughout the volume, are consistent with predictions of the Mills model, CQM, of populating fractional quantum states in H-atoms via catalytic resonant transfer processes. In particular CQM predicts that O-2 can act as a catalyst for the RT process, and spectroscopy shows the water plasma contain a significant O-2 population. Standard physics models of preferential hydrogen line broadening, developed explicitly for hydrogen/argon mixtures, are clearly not consistent with any aspect of the behavior observed in the water plasma data reported here. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.