This paper presents a detailed study of the effectiveness of Gd-doped Ceria (GDC) barrier layers for preventing reactions between La0.6Sr0.4Co0.2Fe0.8O3 (ISCF) cathodes and Yttria-stabilized Zirconia (YSZ) electrolytes. The GDC barrier layers, which are reasonably dense as produced by a novel process where the GDC and YSZ are co-fired at 1250 degrees C, were varied in thickness from 1.5 to 6 mu m. No measureable Sr transport occurs across the GDC layer of any thickness for a relatively low LSCF firing temperature of 1000 degrees C, or for thickness >= 4.5 mu m at any firing temperature. Sr was detected at the GDC/YSZ interface for thinner barriers and higher firing temperatures, in the form of a discontinuous SrZrO3 phase. This zirconate phase forms preferentially at pores and grain boundaries in the GDC layer, probably indicating that these were pathways for relatively fast Sr transport. Atom probe tomography results indicated segregation of impurities to grain boundaries near the GDC/YSZ interface.