Beryllium will be utilized as the neutron multiplier in the ITER breeding blanket. As part of Japanese contribution of the water-cooled blanket design, preliminary study was performed to investigate beryllium breakaway reaction, which is one of safety issues of ITER under high temperature conditions. Thermogravimetric measurements in the temperature range 550 - 750 degC were carried out under helium gas flow containing water vapor of 7.6 and 0.76 Torr. The test samples were prepared from commercially available hot-pressed and hot-rolled beryllium plates. Characterization of the surface reaction product was performed by macro and microscopic observations, and X-ray diffraction analysis. Linear and parabolic rate laws were found for the dominant reaction steps in the preceding period of the breakaway reaction. Microstructure of the surface reaction layer formed in all exposure conditions revealed brittle structure, which was composed of blister and microcracks. The rate equations were obtained for the preceding step of breakaway reaction. (J.P.N.).