Abstract Current lithium ion battery technology makes use of flammable liquid electrolytes and so the development of solid ceramic electrolytes for the next generation of all-solid-state batteries can offer a safer alternative. However, the lithium diffusion behaviour in these solid electrolytes is not yet well characterised, despite the importance of this information for optimising cell performance. Similarly, the transport properties at the metal anode interface are critically important, but not well understood. We propose a methodology for obtaining lithium diffusion coefficients of bulk solid ceramic garnet-type Li₇La₃Zr₂O₁₂ (LLZO) electrolytes by coupling dense pellets with isotopically labelled lithium metal, followed by analysis with focused-ion-beam secondary ion mass spectrometry. We report room temperature lithium diffusivities of 2–8 × 10⁻¹³ m² s⁻¹ for doped LLZO using an estimate of the lithium diffusion length in good agreement with electrochemical impedance spectroscopy. Simultaneous detection of positive and negative secondary ion species by SIMS enables correlation of layered interfaces consisting of metallic lithium, corrosion/surface degradation products and bulk LLZO during depth profiling. Charging of the ceramic during ion sputtering is investigated and shown to have a minimal effect on the obtained lithium isotopic fractions in the current setup. Additionally, the effect of the presence of corrosion products at the surface of garnets as a result of air-exposure is investigated. This method could be extended to any Li-metal stable solid electrolyte, or with a reactive solid electrolyte coupled with a stable interlayer. As such, this work sets the basis of a methodology for further quantitative diffusion analyses for Li-conducting solid ceramic electrolytes and their interfaces with electrodes, as used in both solid-state lithium batteries and hybrid systems coupling solid and liquid electrolytes.