Published in Petroleum Transactions, AIME, Volume 219, 1960, pages 75–80. Abstract Two equations are compared for calculating imbibition rates of water into porous media. A linear relation was found to apply when the square of the volume of water imbibed was plotted against time for water imbibing in natural rock materials. The effect of absorbed organic materials on surface wettability was determined from measurements of rate of imbibition, water saturation and water permeability. Introduction An understanding of the mechanisms by which water displaces oil from porous media requires an understanding of the role of capillary forces in the displacement process. The capillary forces are determined by properties of the water-oil-solid surfaces. Consequently, some measure of the surface properties of reservoir rocks is necessary if displacement behavior in oil reservoirs is to be predicted from laboratory measurements. Several experimental procedures have been proposed to assign quantitative wettability indexes to reservoir rock surfaces. The most recent of these proposals are those of Bobek, Mattox and Denekas and Amott. These indexes are designed to show a continuous variation from the preferential oil-wet to the preferential water-wet systems. They require measuring some property of the rock which is a function of surface wettability. The quantities are measured on unaltered core material and compared with values obtained for known oil-wet and water-wet extremes on the same material. These methods are useful but are semi-empirical in nature. they have the disadvantage that the measured quantities may be functions of other variables in addition to surface wettability. Some understanding of the behavior of water-oil-rock systems may be obtained from studies of the simpler water-air-rock systems. Surfaces of a given wettability are more easily maintained for the latter. Also, certain approximations can be made for the flow of water and air which are not justified for water-oil flow. The theoretical and experimental work discussed in this paper is part of a study of the role of capillarity in the dynamic displacement of air by water. The expectation is that observations for the water-air system will lead to generalizations which are applicable to water-oil systems.