A composition for treating a keratin based substrate that includes a cosmetically acceptable medium containing a water-soluble interjacent complex. The water-soluble interjacent complex includes a first water-soluble polymer and a second water-soluble polymer formed by polymerizing one or more water-soluble monomers in the presence of the first water-soluble polymer. The water-soluble interjacent complex is characterized in that it forms a solution in water that is free of insoluble polymer particles. The water-soluble interjacent complex is used in a method of treating a keratin based substrate, whereby a cosmetically acceptable medium is applied to the substrate and contains from 0.1-20% by weight of the water-soluble interjacent complex.