Grain boundary (GB) mass transport, and chemistry exert a pronounced influence on both the performance and stability of electrodes for solid oxide electrochemical cells. Lanthanum strontium cobalt ferrite (LSCF6428) is applied as a model mixed ionic and electronic conducting (MIEC) perovskite oxide. The cation-vacancy distribution at the GBs is studied at both single and multi-grain scales using high-resolution characterization techniques and computational approaches. The accumulation of oxygen vacancies (V⋅⋅O) in the GB region, rather than necessarily at the GB core, results in an enhancement of the oxygen diffusivity by 3 – 4 orders of magnitude along the GBs (Dgb). At 350 °C, the oxygen tracer diffusion coefficient (D*) is measured as 2.5 × 10−14 cm2 s−1. The Dgb is determined to be 2.8 × 10−10 cm2 s−1 assuming a crystallographic GB width (δcrystal) of 1 nm, and 2.5 × 10−11 cm2 s−1 using a chemically measured δchem of 11.10 nm by atom probe tomography (APT). The origin of the concomitant changes in the cation composition is also investigate. In addition to the host cations, strong Na segregation is detected at all the GBs examined. Despite the low (ppm) level of this impurity, its presence can affect the space charge potential (Φ0). This, in turn, will influence the evolution of GB chemistry.