The effect of steam, NaCl. or KCl on potassium pentaborate was investigated at high temperature. The structure changes of KB5O8 in high temperature water steam were studied by a self-designed reactor, XRD, and Raman spectroscopy. The reaction between KB5O8 and water steam at solid state and molten state was confirmed. Ratio of B2O3 to K2O in reaction product would be decreased by high temperature steam. The melting diagrams of binary systems KB5O8-MCl(M=K, Na)were obtained by means of DTA and X-ray diffraction analysis. It is found that KB5O8,-MCl(M=K, Na)systems are of simple eutectic type with eutectic points (X-KCl = 50%, 742 degrees C and x(NaCl) = 20%, 689 degrees C), respectively. The addition of NaCl or KCl can not affect the structure of KB5O8
