•(NH4)3ScF6 solubility decreases with increased NH4F concentration.•(NH4)3ScF6 solubility decreases with increased alcohol concentration.•(NH4)3ScF6 solubility relates to system dielectric constant for different alcohols.•(NH4)3ScF6 transforms into NH4ScF4 in water and other phases in low fluoride media.•(NH4)5Sc3F14 is a possible transformation product of (NH4)3ScF6 in 0.5 mol/L NH4F. This paper investigates the phase equilibria of ammonium scandium fluoride phases in pure water, aqueous NH4F solutions and in mixtures of NH4F and alcohols. The solubility of the solid phases was determined at 25 °C. It was observed that (NH4)3ScF6 transforms into NH4ScF4 when contacted with pure water for 24 h. Solid phase transformation was also noted in NH4F solutions of concentration ≤ 0.5 mol/L. At NH4F concentrations ≥ 0.8 mol/L, no phase change has been observed, and the solubility of (NH4)3ScF6 decreases with increasing NH4F concentration due to the common ion effect. In NH4F-alcohol mixtures, the solubility of (NH4)3ScF6 decreases in the order: methanol > ethanol > 1,3-propane-diol > 2-propanol. This aligns with the decrease in the effective dielectric constant of the resulting solvent mixture, which is very similar for NH4F-alcohol mixtures formed by ethanol and 1,3-propane-diol.