The formation of 3-rings was monitored by the boron percentages participating in 3-rings (B ring ), as shown by the red lines in Fig. (right-hand axes). An increase in B ring during the cooling process from 1500 to 300 K was found in a-La30 glass. Because the increase in B ring is greater than the change in B 4 , 3-ring formation is attributed to the interconnected reaction of three B 3 s. At higher temperatures, it is suggested that the formation of 3-rings is suppressed by the active atomic motion. For the glasses of other compositions in this study, it is difficult to discuss the changes in B ring during the cooling process due to the large fluctuations resulting from the small amount of boron in 3-rings. The percentages of B assigned to the structural units as indicated in Fig. were evaluated as a function of temperature; Fig. shows the percentages of B belonging to isolated monomer, dimer, and trimer units. Note that a small amount of B with isolated units-less than 10 %-was found in the a-La30 glass, because most of the B belongs to polymers. At higher temperatures, thermal excitation may allow more dispersed structural units, and it is possible that unstable structures comprising the units surrounded by blue dashed lines in Fig. may be formed.