Contamination at surfaces and interfaces is known to cause lowered damage threshold in high power laser optical components; therefore, it is important to be able to identify and measure the amount of adsorbed species. A technique has been developed for detecting water and other adsorbed molecules on optical surfaces at incident intensities below the damage threshold. A pulsed laser is focused onto the optical surface of interest in a UHV chamber. The laser energy absorbed in the surface layer causes desorption of the contaminants. Detection and identification of the desorbed species are via a quadrupole mass analyzer. As water and other contaminants such as hydrocarbons are strong optical absorbers in the 2.8- to 3.8-μm wavelength region, an HF/DF laser was chosen for the initial investigation. H₂O, OH, and other species have been observed from both coated and uncoated optical surfaces. Surface mapping of adsorbed H₂O with a 121μm focal spot shows considerable variation with spatial position. This effect may be associated with surface microcracks and other defects which may precipitate laser damage. Possible applications are characterization of laser optical components and laser cleaning of optical surfaces prior to film deposition.