Emulsions consist of mixtures of immiscible liquids where one liquid is finely dispersed within the continuous phase of another, They are generally not thermodynamically stable : the dispersion tends to separate over time. Aqueous emulsions, widely used in food, pharmaceutical, and many other industries, are often stabilized by block copolymers containing alternating hydrophilic and hydrophobic segments (typically based on ethylene oxide/ propylene oxide diblock and triblock systems) that penetrate into the oil and aqueous phase, respectively(1,2). Here we describe a conceptually new type of emulsifier whose hydrophobic blocks are formed spontaneously and reversibly by the complexation of hydrophilic segments, thereby allowing the stabilizing properties of the system to be switched on and off. We illustrate this approach using a comb-type graft copolymer containing a poly(methacrylic acid) backbone and short grafts of poly(ethylene glycol), The uncomplexed polymer is hydrophilic, but acidic conditions induce the formation of hydrogen-bonded hydrophobic complexes between parts of the backbone and the grafts. As a result, the grafted copolymer forms alternating blocks of hydrophilic (uncomplexed) and hydrophobic (complexed) segments that stabilize acidic emulsions, An increase in pH suppresses complex formation and thus leads to the breakup of the emulsion. Emulsion tests show that although the performance of the grafted copolymers is not Vet competitive with existing emulsifiers, this approach provides an efficient strategy for the design of fully reversible emulsifiers.