The water activities of unsaturated solutions of potassium perchlorate + poly(ethylene glycol) + water at 298.15, 308.15 and 318.15 K and densities and viscosities at 288.15, 298.15 and 308.15 K were measured experimentally. The concentration range of the solutions was from 0.2 to 0.8 mass % of poly(ethylene glycol) with an average molecular weight of 4000 and from 0.25 to 1.25 mass % of KClO4. The water activity data show that in the PEG–salt–water system, KClO4 has a significant effect on the water activities and the vapour pressures; a slight increase in water activities with increasing temperature was also observed. Density and viscosity decrease as temperature increases and both properties increase as KClO4 and PEG-4000 concentrations increase. The electrolyte and polymer non-random two liquid models, extended for the representation of water activity, excess molar volume and dynamic viscosity, were applied. Good agreement between the experimental and correlated data was obtained, since for the ternary system, the average absolute deviations calculated at all temperatures studied for the water activity, density and viscosity were 0.0067, 0.72 and 2.91%, respectively.