Microdifferential thermal analysis (mu-DTA), X-ray diffraction (XRD) and infrared (IR) spectroscopy were used for the first time to investigate the liquidus and solidus relations in the KPO3-Y(PO3)(3) system. The only compound observed within the system was KY(PO3)(4) melting incongruently at 1033 K. An eutectic appears at 13.5 mol% Y(PO3)(3) at 935 K, the peritectic occurs at 1033 K and the phase transition for potassium polyphosphate KPO3 was observed at 725 K. Three monoclinic allotropic phases of the single crystals were obtained. KY(PO3)4 polyphosphate has the P2(1) space group with lattice parameters: a = 7.183(4) Angstrom, b = 8.351(6) Angstrom, c = 7.983(3) Angstrom, beta = 91.75(3)degrees and Z = 2 is isostructural with KNd(PO3)(4). The second allotropic form of KY(PO3)(4) belongs to the P2(1)/n space group with lattice parameters: a = 10.835(3)Angstrom, b = 9.003 (2) Angstrom, c =10.314(i) Angstrom, beta =106.09(7)degrees and Z = 4 and is isostructural with TlNd(PO3)(4). The IR absorption spectra of the two forms show a chain polyphosphates structure. The last modification of KYP4O12 crystallizes in the C2/c space group with lattice parameters: a = 7.825 (3) Angstrom, b = 12.537(4) Angstrom, c =10.584(2) Angstrom, beta = 110.22(7)degrees and Z = 4 is isostructural with RbNdP4O12 and contains cyclic anions. The methods of chemical preparations, the determination of crystallographic data and IR spectra for these compounds are reported.