The impact of powdered oxides of Mg, Ca, La, Ti, Al or their nitrate salts on the properties of MeOx/Al2O3/Al composites obtained via the hydrothermal treatment (HTT) of Al-based blends put in a closed space of the die allowing the free water vapor access has been studied. Additives were found to affect the process of aluminum oxidation mainly through variation of the density of formed aluminum hydroxide layer. In some cases, incorporation of dopant cations into the structure of alumina formed after calcination of composite in air was revealed by 27 Al MAS NMR. Variation of the Al oxidation degree and associated change of the solid phase volume in a closed die caused by dopants affects also the pore structure and texture of composites determining the mechanical strength of produced monoliths. The processes of the redistribution of soluble additives between the solid phase and autoclave solutions were revealed and their consequences are considered.