Water activity values of ternary aqueous sodium potassium tartrate (Na-K Tar) + poly (ethylene glycol) or poly (propylene glycol) solutions were measured using the isopiestic method at T = 298.15 K. Also the water activity for corresponding binary aqueous sodium potassium tartrate (Na-K Tar) solutions was measured at T = (298.15, 308.15, 318.15) K. The obtained water activity values for binary aqueous sodium potassium tartrate (Na-K Tar) system were successfully fitted with the local composition and Pitzer models. The mean activity coefficients for the studied binary salt solutions have been calculated. The thermodynamic behaviour of aqueous {(Na-K Tar) + poly (ethylene glycol) or poly (propylene glycol)} solutions was discussed on basis of water, polymer and salt interactions. (C) 2016 Elsevier Ltd. All rights reserved.