The search for the appropriate demulsifier for a water-in-crude oil emulsion is generally carried out through the well-known "bottle test" technique. A comprehensive approach is proposed here, which is based on earlier work on the rate of separation of surfactant- water- hydrocarbon mixtures, where it was found that "optimal" type III systems, in Winsor's nomenclature, exhibits the fastest separation rate. It is shown that the rules developed for microemulsion optimization apply to the selection of the demulsifier. They allow one to take into account; the variation of salinity, temperature, crude-oil type, and so forth and provide an explanation for the demulsifier concentration effect. Interfacial tension measurements have been carried out. They show that the kinetics of demulsifier adsorption at the water-crude oil interface (or alternatively the Gibbs elasticity modulus) is correlated to the phase separation rate, as already described in the literature: the higher the kinetics, the faster the separation rate.