In the ternary system M2O3-K2O-P2O5 a partial system NdPO4-KPO3-Nd(PO3)(3) was investigated by thermoanalytical methods, X-ray powder diffraction and microscopy in reflected light and its phase diagram was proposed. In the system a double metaphosphate KNd(PO3)(4) appears. The compound forms in a side system Nd(PO3)(3)-KPO3. It has been established that the KNd(PO3)(4) and NdPO4 form a section, which is a real system only in the subsolidus region. It is found, that a ternary peritectic (tp similar to 885degreesC) and a ternary eutectic (t(E) similar to 688degreesC) occur in the partial system NdPO4-KPO3-Nd(PO3)(3).