Extensive density measurements were performed with Mixtures of 12 degassed (dead) crude oils with respective oilfield brines (formation waters). All unprocessed samples were collected directly from wellheads and contained only indigenous surfactants, such as asphaltenes and fine solids. Nonzero excess densities and excess thermal expansions (evaluated on the assumption of quasi-binary water-oil mixtures) were observed for water cuts in the range from X = 0.4 to 0.6 at all studied temperatures T = 5-50 degrees C. We suggest that these results are due to the formation of a dense asphaltene-mediated "middle phase" in the studied w/o dispersions. This suggestion is substantiated by plotting T-X phase diagrams which topologically strongly resemble those conventionally observed in some standard Winsor III-type systems. The formation of a complexly structured "middle phase" has been directly verified by preliminary visual/microscopic studies of a phase separation in a crude oil-wate mixture. (c) 2004 Elsevier Inc. All rights reserved.