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In this study, the formation mechanism of strontium hexaaluminate (SrAl 12 O 19) in Al 2 O 3 –SrO and Al 2 O 3 –SrO–ZrO 2 composites was investigated. Strontium oxide (SrO) powder was added to initial suspensions in various quantities. High-purity submicron powders were used during the experiments. Powder compositions were thermally treated in the temperature range of 900°С–1500°С for 5 h. The stepwise character of SrAl 12 O 19 formation was established by X-ray phase analysis. The SrAl 2 O 4 intermediate compound formed in the temperature range of 900°С–1200°С. The onset of SrAl 12 O 19 formation corresponded to 1200 °C. Formation of SrAl 12 O 19 was completed in the temperature range of 1400°C-1500°С. After sintering at 1300°С–1500°С for 1 h, the samples were analysed. The density of materials indicated a decreasing trend with an increase in the amount of SrO. The density of alumina ceramics sintered at 1500 °C was 3.65 ± 0.2 g/cm3. However, alumina ceramics comprised 15 wt% SrAl 12 O 19 and their density was 3.35 ± 0.4 g/cm3. A decrease in the density level is explained by the platelet shape of SrAl 12 O 19 grains, which prevented additional contact points from generating between the matrix grains and led to porosity formation in the materials. The effect of a reduction in the alumina grain size was established with an increase in the content of the initial SrO and SrAl 12 O 19 powders. Scanning electron microscopy found that the SrAl 12 O 19 platelets comprised one to three layers fused together along the basal planes. Zirconia grains inhibited the lateral growth of the SrAl 12 O 19 platelets. [ABSTRACT FROM AUTHOR]