Abstract: In accordance with wall of well-bore instability induced by bound water on clays, Infrared spectrum is utilized to determine the types and boundary line of bound water on clays, and the information about chemical-bonding in water and interaction between water and clay-mineral. The results show that
clay-minerals adsorb water when they interact with water.The O-H's expanded and contracted frequency v will reduce along with adsorbed water content growth in hydro-clays.Theprocess that O-H's expanded and contracted frequency vchanges along with relative vapor pressure p/pshas the charac-
teristics - fast first, slow then, fast again, and steady forever.There are two clear turning points which are p/ps=0.9, 0.98,the two turning points are the special humidity of clay-water system. The relative vapor pressure 0.9 is the boundary line between the strong bound water and the loose bound water(os
motic bound water) on clays, the relative vapor pressure 0.98 is the boundary line between the loose bound water and free water on clays. The results are identical with Ion exchange.

Key words: clay, infrared spectroscopy, vibration frequency, bound water, boundary line