Ti–Zr alloy thin films of 20–60 nm in thickness were evaporated on Pt-coated silicon wafers. The films exhibited a layered Ti–Zr depth distribution. The films were then treated hydrothermally in 0.25 M Ba(OH)2 at 150 and 200 °C for 4–8 h. Films treated at 150 °C did not exhibit reflections from the Ba(TixZr1−x)O3 perovskite structure by x-ray diffraction, although a slight barium content was detected by x-ray photoelectron spectrometry. On the other hand, the films treated hydrothermally at 200 °C revealed reflections corresponding to perovskite Ba(TixZr1−x)O3. These films exhibited a homogeneous titanium and zirconium depth distribution, as shown by x-ray photoelectron spectroscopy and Auger depth profiles, proving that either titanium or zirconium ions diffuse during the hydrothemal treatment. The initial Ti–Zr film was completely consumed during the hydrothermal process, leading to a film of homogeneous composition (Ba, Ti, and Zr) up to the interface with the platinum layer.