Equilibrium contact angles are reported for silicone oils (poly(dimethylsiloxane)s) on polymer-coated and uncoated solid-silicon substrates immersed in aqueous electrolyte solutions. Solid-substrate wettability to water ranges from highly hydrophilic to highly hydrophobic based on water/air contact angles. Although silicone oils in air spread completely on all of the Studied substrates, these same surfaces when immersed in aqueous media exhibit finite contact angles against silicone oils that depend strongly on the substrate surface energy. A detailed investigation of the pH influence on the wetting behavior of silicone oil on the solid substrates is pursued where a clear correlation emerges between the changes of substrate-surface zeta potential (zeta) and the oil-wetting behavior on substrates immersed in aqueous solution. Also, the influence of inorganic KCl and CaCl2 electrolytes on the wetting behavior of silicone oils on solid substrates is studied. KCl does not produce a noticeable effect on the wetting behavior of silicone oils. CaCl2, in general, increases surface hydrophilicity, with the exception of the chitin-coated silicon-wafer substrate. For this polymer surface, the oil/water contact angle decreases with increasing CaCl2 concentration, indicating stronger oil wetting. Specific interactions between chitin surface functional groups and calcium ions are confirmed by zeta-potential measurements. Finally, we find that the classic Bartell-Osterhof equation, based on the thermodynamics of ideal wetting, well describes the wetting behavior of silicone oil in water on apolar and moderately polar solid substrates of different functionality, provided that accurate measurements are available for the corresponding air-in-water contact angles and for the equilibrium surface and interfacial tensions. The same Bartell-Osterhof equation yields invalid predictions for contact angles of oils in water on substrates wettable by both liquid phases and for contact angles of water on the substrates studied when they are immersed in silicone oils.