In this study, a copolymer of N,N-dimethylacrylamide and hydroxyethyl-methacrylate was synthesized and hydrophobically modified with about 2 mole % of adamantyl groups. The mixing of this modified copolymer with water-soluble β-cyclodextrin polymers reveals thickening properties. Inclusion complexes formation between adamantyl groups and β-cyclodextrin cavities are responsible of intermolecular associations. Above a given total polymer concentration a phase separation occurs, giving a gel phase topped by a liquid phase. We have stated the phase diagram by analysis of the two phases in equilibrium. It has been observed that the two polymers are present in both phases and that the gel phase is more concentrated than the other one. This phase behavior is an original complex coacervation phenomenom which is due to the specific attractive interactions of the two different macromolecules: inclusion complexes formed between the two neutral polymers.