A critical evaluation and thermodynamic optimization of experimental phase diagrams and thermodynamic properties of the K2O-Al2O3-SiO2 system was performed at 1 bar total pressure. A set of self-consistent thermodynamic functions of all phases in the K2O-Al2O3-SiO2 system was obtained. The liquid phase was described using the Modified Quasichemical Model with the KAlO2 associate component. The set of optimized model parameters obtained for all phases reproduces available and reliable thermodynamic properties and phase diagram data as well as the melt structure of the K2O-Al2O3-SiO2 system within the experimental error limits.