Abstract Many crude oils contain components which lead to the formation of solid films at the interface between the crude oil and water. Film forming ability varies widely among crude oils. It is completely absent in some, although most crudes will exhibit film formation at oil-water interfaces. The effect of these solid films on the displacement of oil by water has been demonstrated in laboratory flow studies using n-hexadecane and water as the immiscible fluids, and stearic acid as the film forming additive. These tests were conducted on highly reproducible glass bead packs. The presence of solid films can measurably increase the amount of oil recovered before water breakthrough. This increase may vary from as high as 52 per cent additional oil when connate water is absent to 14 per cent additional oil when connate water is present. The water production from breakthrough to a producing water-oil ratio of 100 is reduced by the presence of solid films. Decreases may be as high as 64 per cent when connate water is present. Solid film formation has little effect on the total oil recovered by flooding to WOR = 100 when connate water is present. However, in the absence of connate water, the oil recovery is less by about 12 per cent when flooding to the same water-oil ratio. The fact that solid film formation exerts such a pronounced and definite effect on laboratory systems indicates that further film effects in reservoirs where they are absent or enhance their effect in others. Introduction The existing information regarding the solid films which often occur at oil-water interfaces has been previously summarized and methods have been described whereby these films may be investigated in the laboratory. This technique of investigation has been successfully applied to the study of one crude oil-water system. Thus far, however, there has been no direct experimental evidence on the effect which these films display in the displacement of oil by water. The goal of the experiments described here was to demonstrate, under controlled laboratory conditions, the effect of film formation on displacement systems differing only by the presence or absence of films. The system chosen for these tests was n-hexadecane-water. Stearic acid dissolved in the n-hexadecane could be demonstrated to form films, at the n-hexadecane-water interface, which appeared very similar to those observed with natural crude oil-water systems. Moreover, the amount of stearic acid [about 14 parts per thousand] necessary to achieve this film formation was so slight as to have little effect on the n-hexadecane-water interfacial tension. The effects on other properties of the n-hexadecane were almost completely negligible. In order to achieve reproducible packing among the tests and to minimize any other side effects, glass beads were used as the porous medium. Materials The glass beads used were U.S. 60 screen No. 11 grade. Their range of diameter was 0.0095 - 0.0059 in. Their density was 2.45 gr/cc. The normal hexadecane was designated ASTM grade and has an apparent purity of greater than 99 per cent. Eastman reagent grade stearic acid was used. The water for flooding was laboratory deionized water. Previous tests had established this water as free of surface active impurities.