When heated, orthoboric acid undergoes a three step dehydration. The reported transition temperatures vary substantially from source to source.
When heated above 140 °C, orthoboric acid yields metaboric acid () with loss of one water molecule:
Heating metaboric acid above about 180 °C eliminates another water molecule forming tetraboric acid, also called pyroboric acid ():
Further heating (to about 530 °C) leads to boron trioxide: