We have deposited thin films (3.5, 7.5 and 22 nm) by atomic layer deposition (ALD) using HfCl4 and H2O precursors at 350 °C. Growth, thermal annealing and thermal reoxidation of the thin hafnium oxide layers under controlled ultra-dry oxygen atmosphere were studied using ion beam techniques and isotopic tracing experiments. Secondary ion mass spectroscopy (SIMS) profiling shows that the composition of deposited films is homogeneous with depth and over a large area. RBS and NRA show that the films are under-stoichiometric in oxygen and contain trace chlorine contamination, more pronounced at the film–substrate interface. After oxidation for 20 min in 100 mbar O2 enriched to 99.9% in 18O at 425 °C, nuclear resonance depth-profiling using the 151 keV 18O(p,α)15N narrow resonance, reveals that the main process occurring is exchange between oxygen from the gas and oxygen from the film matrix. However, following a post deposition vacuum or inert gas anneal, the atomic exchange process during thermal reoxidation, in 18O2, is significantly inhibited and limited to the superficial region. We assume a link between this effect and the crystallization of the films previously reported.