A morphous solids belonging to the systems Al2O3–Me2O (Me = Na, K) and Al2O3–B2O3were prepared by nitrate decomposition, introducing boron in the form of boric acid. Crystalline metastable solids with pseudotetragonal symmetry were obtained from thermal treatment at 850° to 900°C for the compositions Al6MexO(9+0.5x)(x≅ 1; Me = Na, K) and Al6‐xBxO9(1x3). The resultant solids were stable only within a difinite temperature range and transformed, with further treatment increases, into stable equilibrium phases. The structures of the metastable phases were examined by X‐ray diffraction and Fourier transform infrared spectroscopy, and both analyses showed a mullite type of framework, inside of which the atomic coordinates were refined in thePbam(no. 55) space group. The present results indicate that these silica‐free mullite structures are stabilized by two different mechanisms: (1) interstitial occupation of bulky cations (Na+, K+) or (2) substitution of B for Al in some of the tetrahed