The effect of systematic changes in the structure of chemicals commonly used in demulsification operations (alkylphenol polyalkoxylated resins and polyurethanes) on the stability and properties of brine-in-crude oil emulsions was assessed experimentally. The relative rates of water separation were characterized via bottle tests and rheometry. Equilibrium interfacial tensions were also measured. Transient changes in drop size distributions were quantified using nuclear magnetic resonance. The phenolic resins promoted coalescence of droplets. Optimum performance was obtained with resins exhibiting intermediate hydrophilicity in a manner consistent with the condition of least emulsion stability for conventional oil-water-surfactant systems. In contrast, polyurethanes promoted flocculation but only slow coalescence. The performance of polyurethanes improved with increase of molecular weight. Phenolic resins and polyurethanes acted synergistically when added simultaneously, rendering water separation rates significantly higher than those observed when they were used individually. Polyurethanes aided sedimentation of water at moderate concentrations (ca. 200 ppm) by "bridging" nearby droplets, but they retarded coalescence when added at significantly higher concentrations, even when phenolic resins were present. Plausible mechanisms for demulsification that are consistent with these findings are proposed and discussed.