The variation of outgassing rate with pumping time for water vapor from unbaked metal surfaces has been shown by several authors to depend on the ratio of pumping speed to exposed surface area, the time of initial exposure to the atmosphere at a known humidity, the ambient temperature, the thickness and porosity of the oxide layer on the metal surface, and the pretreatment or conditioning of the surface. The outgassing behavior differs radically from that for plastics and elastomers, and many authors have attempted to explain this behavior on the basis of theoretical models for the molecular kinetics. These models will be reviewed and evaluated. The present author will suggest a new model based on rapid physical adsorption and desorption in the oxide layer, with heat of adsorption in the neighborhood of 10 kcal/mol (corresponding to two hydrogen bonds), during atmospheric exposure followed by a slower (low probability) transition from a physisorbed precursor state (with activation energy of about 5 kcal/mol corresponding to breaking of one of the hydrogen bonds) to a "weak chemisorption state'', involving rotation and wobble about the remaining single hydrogen bond until an exposed pair of electrons associated with the water molecule has been captured in the positive field of a nearby metal ion without necessarily breaking the hydrogen bond to the oxide ion. Outgassing then involves the desorption of the undissociated water molecule from this bound state to the gas phase with an energy of desorption ranging from 20 to 30 kcal/mol.