Precise investigation of part of the phase diagram of KF-Al2O3 system was performed in an experiment combining different techniques. Solidified mixtures of KF-Al2O3 were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO2, K2Al22O34, α-K3AlF6, KAlF4, and K2Al2O3F2. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF-Al2O3 with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K2Al2O3F2.