Understanding aqueous corrosion reactions at high temperatures (280-350°C) and the correlated hydrogen pickup into Zr fuel cladding alloys is crucial to the safe operation of PWRs and to increasing the burnup of the fuel. The mechanism by which hydrogenic species from the cathodic reaction migrate through the nanostructured oxide on the cladding is very difficult to study by conventional techniques. We will report the development of a method using a NanoSIMS instrument to characterise with sub-micron resolution the 3D distribution of deuterium in Zircaloy-4 samples oxidised in high-temperature heavy water. The topography of the sputtering carter, sputtering rate and depth resolution have all been calibrated by Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) analysis. Two analysis directions, depth-profiling and cross-sectional, were used on the same samples to ensure we understand possible imaging artefacts during sputtering of the complex microstructure in these samples. The results show that the roughness of the sputtering surface is small enough to be ignored for 3D data reconstruction and the depth resolution of the 2H- signal in the oxide is about 11-15 nm. However, the effect of deuterium outgassing and surface diffusion during Cs+ primary ion bombardment should be considered because it results in a degraded lateral resolution, which distorts the apparent deuterium distribution. With the combination of this method and detailed microstructural information obtained by SEM and Transmission Electron Microscopy (TEM) can provide unique mechanistic information on the mechanisms of hydrogen uptake during the corrosion of zirconium in PWR environments.