The phenomenon of the sharp increase of activity in reactions of deep oxidation found for the manganese oxide-alumina system calcined at 900-1000degreesC was named as thermoactivation effect. This effect was shown to be associated with phase transformations in this system. Peculiarities of phase transformations in the MnOx/Al2O3 system after its calcination at 950 degreesC for 1-10 h were studied with XRD, TEM, XPS methods; the catalysts activity in deep oxidation of n-pentane was measured in a flow-circuit setup. It was found that a set of phase transformations occurred in the system, starting from the formation of the metastable nonstoichiometric manganese-alumina cubic spinel (after 1-2 h calcination) and nonequilibrium solid solution of Mn3+ ions in the structure of gamma-Al2O3 which decomposes producing high-temperature aluminas modified by manganese ions and particles of imperfect beta-Mn3O4 phase doped with aluminium ions. Increase of the calcination time to 10 h and more results in the formation of biphasic system composed of alpha-Al2O3 and nanocrystalline beta-Mn3O4 modified by Al3+ ions. These transformations are associated with the maximum activity showed by catalysts calcined for 5-6 h in the reaction of n-pentane combustion. (C) 2002 Elsevier Science B.V. All rights reserved.