Phase transition study of the superionic system Li3BO3 : x Li3PO4 with x = 0.0, 0.33, 0.50, 0.67, 1.0 1.5 and 2.0 has been done in the temperature internal of 500 K to the melting point of the studied materials. This study has been done by molar magnetic susceptibility (cM) and dielectric constant (K) measurements. The temperature around which anomaly appears in cM and log K against absolute temperature plots, is the phase transition temperature and is also confirmed by electrical conductivity (s) and Seebeck coefficient (S) results. Studied materials remain diamagnetic throughout the measurement.