Understanding the nature of cation diffusion through the reaction barrier layer provides important information on the long-term stability of solid oxide fuel cells. In this work, model LSCF/GDC/YSZ diffusion triplets were fabricated by pulsed laser deposition. To understand the role of the barrier microstructure on cation transport, epitaxial GDC barrier layers were prepared on (100) and (111) single crystal YSZ substrates. The GDC films sudcessfully prevented severe Sr diffusion into YSZ electrolyte. However, an active Zr transport was observed especially on the (100)-oriented GDC interlayer which is accompanied with severe pore formation. Long-term annealing at high temperature revealed nanosized SrZrO3 grains at the LSCF/GDC(100) interface. SIMS and TEM analyses indicate that Zr is primarily transported through the dislocations. By varying the GDC film thickness systematically between 10 nm and 1.0 mu m, we demonstrate by high-resolution reciprocal space mapping that the strain thickness levels in GDC films are different. The (111)-oriented GDC film is strained to the YSZ for thicknesses up to 200 nm, which is several times greater than the (100)-oriented GDC films indicating that strained layers inhibit the Zr transport. The presence of a network of dislocations in GDC provides the fast diffusion pathway for Zr.