A family of TiO2–Al2O3 multilayers (Λ=2–72 nm TiO2/7 nm Al2O3) is sputter deposited on fused silica substrates, sequentially annealed at 973 and 1273 K, and analyzed by x-ray diffraction. The goal is to examine crystallization behavior upon annealing at temperatures at which thermodynamically stable mixed-cation phases should not form. The results show: (1) After the 973 K anneal, films with Λ=18–72 nm TiO2/7 nm Al2O3 weakly crystallize with a preferred (110) rutile orientation. In addition, enhancement of (200) rutile diffraction increases with increasing TiO2 layer thickness. (2) Significant crystallization occurs in films after the 1273 K anneal. In films with Λ=36–72 nm TiO2/7 nm Al2O3, a metastable pseudobrookite phase, Al0.95Ti2.05O5, crystallizes along with (110)r. However, only rutile TiO2 and α-Al2O3 crystallize in films with thinner TiO2 layers. An architecture-sensitive crystallization model is presented in which the first step common to all architectures is diffusive amorphization of TiO2 by Al2O3 at 973 K to produce an amorphous mixed-cation solid solution, a-ATO. As the reaction interface sweeps through a TiO2 layer in architectures with thicker TiO2 layers, (200)r preferentially crystallizes from a-ATO because (200)r planes can accommodate Al species with the least rutile lattice strain. We suggest that (200)r provides crystallization sites for Al0.95Ti2.05O5 at 1273 K.