Slab optical waveguide (SOWG) spectroscopy is the optical absorption spectroscopic techniques using a thin planer waveguide as an internal total reflection medium. Remarkable features of this technique are the selectivity to the surface and the high sensitivity. As a new field of the application, an experimental dew point measurement system based on the SOWG technique was tested for moisture analysis in high purity gases. It was consisted of a thin glass plate with the thickness of 0.1 to 0.5 mm, optical detecting components and a Peltier cooler. Our results show that this system has the good sensitivity and quick response.