Ultrathin BaZr0.8Ti0.2O3 films (t < 30 nm) on SiOx/Si substrates were obtained by means of aqueous chemical solution deposition (CSD). Though the precursor crystallized into cubic perovskite powder at 600 °C, ultrathin films only crystallized at 950 to 1000 °C, even after addition of excess Ba to compensate for loss of Ba. Films with thickness above 100 nm, on the other hand, crystallized readily around 650 °C. The crystallization is related to film thickness, affecting the crystallizationactivation energy, and to silicate formation by reaction with the substrate, exerting its largest influence in ultrathin films. Barium deficiency, silicate formation, carbonate secondary phase and the high activation energy for crystallization resulted in the amorphous nature of the ultrathin films, which strongly affects the observed k value ( 15). The paper contributes insights with implications for the application of BaZr0.8Ti0.2O3 as an alternative high-k gate dielectric.