K2CaSiO4 or K2O × CaO × SiO2 has been reported to be a major constituent of residual materials that are of interest for the field of applied mineralogy, including ashes from biomass combustion as well as fertilizers produced from the residues of oil-shale industry or from steelmaking slags. Unfortunately, contradictory results concerning basic crystallographic and physicochemical data of this compound have been described in the literature. In a series of solid-state reactions we tried to prepare this material for a more detailed investigation. The temperature regime for the subsolidus synthesis experiments was selected according to previous studies where the occurrence of this so-called “1:1:1” phase had been reported. The samples were characterized by thermal and X-ray fluorescence analysis as well as X-ray powder diffraction. Our results indicate that “K2CaSiO4” does not exist as a crystalline phase in the ternary system K2O–CaO–SiO2 and that the 1:1:1 compound, mentioned in earlier studies is actually misinterpreted K2Ca2Si2O7. Furthermore, the start of melt formation of an oxide mixture with composition K2O × CaO × SiO2 has been determined to be 1170°C, which is dramatically lower than the value of 1630°C mentioned in the only available comprehensive but more than 80 years old phase analytical study on potassium calcium silicates.