A family of TiO2-Al2O3 multilayers (Lambda = 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 Lambda = 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 Lambda = 36-72 nm TiO2/7 nm Al2O3, a metastable pseudobrookite phase, Al0.95Ti2.05O5, crystallizes along with (110)(r). However, only rutile TiO2 and alpha-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), preferentially crystallizes from a-ATO because (200), planes can accommodate Al species with the least rutile lattice strain. We suggest that (200), provides crystallization sites for Al0.95Ti2.05O5 at 1273 K. (c) 2006 American Vacuum Society.