A series of 18O tracer in-diffusion experiments have been conducted on state-of-the-art metal organic chemical vapor deposition–grown Pb(Zr,Ti)O3 (PZT) films. Prior to 18O incorporation, samples were preequilibrated in 16O2 at the same temperature and total pressure used for the following 18O2 isotope exchange anneal. The final 18O depth profiles were obtained using secondary ion mass spectroscopy (SIMS), which provides a depth resolution of 3–5 Å. The SIMS results indicate that the 18O tracer diffusivity varies as a function of depth from the PZT film surface in a way that is inconsistent with a combined grain boundary and lattice diffusion mechanism. Based on a previously-developed model for point defect equilibrium in metal titanate thin films, simulations of 18O tracer diffusion into PZT thin films that are consistent with the experimental results are described.