The precise structural characteristics of the amorphous solids that are generated during the dehydration of crystalline MgHPO4 · 3H2O (newberyite) are reported. The results show that a dramatic change in the distribution of phosphate anions (i.e., chains of corner-linked PO4 tetrahedra) occurs during the process of dehydration and that anions up to 13 PO4 tetrahedra in length are formed. The distribution of phosphate anions formed at intermediate stages of the dehydration process was found to be in quantitative agreement with the theory of Parks and Van Wazer.