The hydroxide precipitation method is used to produce initial materials for subsequent synthesis of alumomagnesium spinel. Using the IR spectroscopy and x-ray phase analysis methods, the processes of interaction between the materials obtained, phase transformations, and structure formation in alumospinel under thermal treatment are investigated. It is established that the spinel formation process passes via an intermediate phase, i.e., crystal hydrate Mg5Al4O11 ċ 15H2O, which in heat treatment transforms into aluminomagnesium spinel. With increasing temperature its structure is perfected and its quantity increases significantly. The formation of this phase ends at a temperature of 1300 – 1350°C.