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A coarsening treatment in an HCl containing atmosphere was performed to stabilize the porosity of Al 2 O 3 /ZrO 2 composites up to elevated temperatures. There is minimal shrinkage during the reactive coarsening treatment and the porous microstructure is stable during subsequent heat treatments. The reaction bonding of aluminum oxide (RBAO) process was used to fabricate the samples. Coarsening was investigated as a function of initial density (40–60 % TD) and coarsening temperatures (1250–1400 °C). After the HCl coarsening step, samples were sintered in air to determine the effect of coarsening treatment on pore stability. Up to the temperature of the coarsening treatment no further densification is observed. Above the HCl coarsening temperature densification is significantly retarded. Thus, it is shown that the porosity in RBAO can be stabilized by heat treatment in a 10 vol % HCl containing atmosphere. During the coarsening treatment in HCl at 1300 °C, 1350 °C and 1400 °C little densification was observed (Δ ρ ≤ 2 % TD) in comparison with the densities that result from sintering reaction bonded samples in air at 1350 °C (Δ ρ ∼ 5–8 % TD). Therefore, the HCl coarsening is a promising approach to adjust a desired level of porosity for high temperature applications of porous materials. It is postulated that at a given density, the coarsened materials should exhibit increased strength compared to un-coarsened materials due to increased contact area between the particles. [ABSTRACT FROM AUTHOR]