The principal purpose here is to discuss why one to two molecular layers of adsorbed water is the proper amount to consider as part of the rock volume for laboratory porosity measurements in order to best describe the reservoir porosity, and in addition, to describe practical laboratory techniques for conditioning clay-bearing reservoir rock samples so these measurements can be made. Introduction Most oil and gas sands contain clay minerals to some degree, and usually several types of clay are present. Water-sensitive reservoirs and problems with clay are encountered in all geographical areas, but in the U. S. they are particularly pronounced in the Gulf Coast and in California and Wyoming. An important use of core analysis data is in the calibration of down-hole logging tools whose responses are interpreted in terms of rock properties (porosity, density, formation factor, etc.). It is therefore desirable for the laboratory values to be as representative as possible of their respective in-situ rock properties. The effect of overburden stress is properties. The effect of overburden stress is recognized to be an important aspect of such representation. but is not the subject of this paper. One of the important obstacles to true representation is encountered when significant amounts of clay minerals (especially montmorillonites) are present in the reservoir rock. The difficulty lies in distinguishing or separating pore water from the proper amount of clay mineral non-liquid water. Core analysts routinely retain (OH) - clay lattice water (lost above 570 degrees F) during the course of measuring the rock properties in the laboratory. However, the last few molecular layers of adsorbed water should also be retained. This portion of the adsorbed water is part of the clay mineral and is not part of the continuous pore-water phase. It is referred to in the literature as nonordinary or nonliquid water. The adsorbed water is fairly tightly held and is more difficult to remove than pore water. However, practically all clay-mineral-adsorbed water, as well as all pore water, is removed from cores when dried in an unhumidified oven at 180 degrees F, the method commonly used to dry core samples. The necessity to control the relative humidity as well as the temperature of ovens in order to remove pore water and retain adsorbed water is clearly shown by published desorption isotherm data, and is easily demonstrated by dehydrating montmorillonite in a humidity-controlled oven. Our purpose here is 1 to discuss why one to two molecular layers ofadsorbed water is the proper amount toconsider as part of the rock volume for laboratoryporosity measurements in order to best describethe reservoir porosity; 2. to describe practical laboratory techniques bywhich clay-bearing reservoir rock samples canbe conditioned to contain one to two layers ofadsorbed water on their clay mineral surfacesfor porosity and permeability measurements;and 3. to show how laboratory data measured withone to two layers of adsorbed water on thehydratable clay mineral surfaces compare withdata measured with essentially all the adsorbedwater removed. JPT P. 800