Abstract Effects of chain association on the demixing behaviour of ternary solutions containing a polyetherimide in a solvent-non-solvent mixture are investigated. Time evolution of the temperature of demixing of the solutions is observed over 1 month. It is ascribed to a slow dissociation process involving charge transfer and hydrogen bonding complexes in the system. The observed demixing curves are characterized by high values of the precipitation threshold. A model based on the Flory-Huggins theory of polymer solutions is developed to explain this effect, and an intermolecular association parameter is introduced into the discussion. Fairly good agreement is obtained between calculated and experimentally found demixing curves. The fraction of associated electron donor or acceptor groups of a polymer chain is predicted by the model.