Heterophase boundaries can offer fast transport paths in solid electrolyte materials. In recent studies an enhancement of the ionic conductivity was indeed observed in micro-/nanoscaled Y(2)O(3)-stabilised ZrO(2) (YSZ) composites and hetero multilayers of thin films. As space charge regions can be neglected due to high charger carrier concentrations, we assume that strain and microstructural changes at the heterophase boundaries are responsible for the observed conductivity effects. In order to obtain independent information on the role of heterophase boundaries for fast transport in strained solid electrolytes, systematic measurements of the (18)O-tracer diffusion coefficient in nanoscaled YSZ/Y(2)O(3) multilayers were performed. Multilayer samples were prepared by Pulsed Laser Deposition (PLD) on (0001) Al(2)O(3) substrates and characterised by X-Ray Diffraction (XRD), Scanning Electron Microscopy (HRSEM) and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). To separate interface and bulk transport from the total oxygen diffusivity of the multilayer system, the (average) thickness of the YSZ-layers in the multilayers was varied from 45 nm to 12 nm. Upon decreasing the thickness of the YSZ layers, respectively increasing the density of parallel interfaces, the total diffusion coefficient of the multilayer system is increased by a factor of 2 compared to bulk YSZ. The experimental results agree well with formerly published data for ionic conductivity measurements. They also support a negligible contribution of partial electronic conductivity in the multilayer.