A low pressure DC discharge formed from a mixture of one of the rare gases Ne, Ar, Kr or Xe plus water vapor is studied, water vapor being only a minor additive to a rare gas. Under some discharge conditions water molecules give hydroxyl molecules OH that produce very intensive UV emission 306.4 nm. It is shown that regarding all rare gases enhanced emission of the OH 306.4 nm band is registered from the discharge of Ar mixed with water vapor. The light production efficiency of the (Ar+H2O)-plasma under study reaches 40–45 LPW that is about (40–45)% of a conventional mercury fluorescent lamp. The experimental data make it possible to propose several ways to increase the efficiency of a (rare gas+water vapour)-discharge plasma as a base for a light source: (1) addition of a second rare gas to the (Ar+H2O)-mixture, (2) usage of TiO2 catalyst to enhance the destruction of water molecules into hydroxyl and atomic hydrogen, and (3) pulse discharge operation which is usually known to give gain in efficiency. The obtained data concerning these ways present possible directions for further investigations and describe predictable results in case of their fulfilment.