Published in Petroleum Transactions, AIME, Vol. 216, 1959, pages 156–162.Paper presented at Fall Meeting of Los Angeles Basin Section in Los Angeles, Calif., Oct. 16–17, 1958. A test is described in which the wettability of porous rock is measured as a function of the displacement properties of the rock-water-oil system. Four displacement operations are carried out:spontaneous displacement of water by oil,forced displacement of water by oil in the same system using a centrifuging procedure,spontaneous displacement of oil by water, andforced displacement of oil by water. Ratios of the spontaneous displacement volumes to the total displacement volumes are used as wettability indices.Cores having clean mineral surfaces (strongly preferentially water-wet) show displacement-by-water ratios approaching 1.00 and displacement-by-oil ratios of zero. Cores which are strongly preferentially oil-wet give the reverse results. Neutral wettability cores show zero values for both ratios. Fresh cores from different oil reservoirs have shown wettabilities in this test covering almost the complete range of the test. However, most of the fresh California cores tested were slightly preferentially water-wet. The changes in core wettabilities, as indicated by this test, resulting from various core handling procedures were observed. In some cases the wettabilities of fresh cores were changed by drying or by extracting with toluene or dioxane; in other cases they were not changed. Contact of cores with filtrates from water-base drilling muds caused little change in wettability while contact with filtrates from oil-base muds decreased the preference of the cores for water. Using this test to evaluate wettability, a study was made of the correlation of wettability with waterflood oil recovery for outcrop Ohio sandstone and for Alundum. Results indicate that no single correlation between these factors applies to different porous rock systems. It is thought that differences in pore geometry result in differences in this correlation.