In this study, we report the micro-structural and chemical evolution of anode grain boundaries and triple phase boundary (TPB) junctions of Ni/YSZ anode supported solid oxide fuel cells. A NiO phase was found to develop along the Ni/YSZ interfaces extending to TPBs in the operated cells. The thickness of the NiO ribbon phase remains constant at similar to 5 nm in hydrogen for operating durations up to 540 h. When operating on synthesis gas, an increase in interphase thickness was observed from similar to 11 nm for 24 h of operation to similar to 51 nm for 550 h of operation. YSZ phases are observed to be stable in H(2) over 540 h of operation. However, for the cell operated in syngas for 550 h, a 5-10 nm tetragonal YSZ (t-YSZ) interfacial layer was identified that originated from the Ni/YSZ interfaces. Yttrium species seem to segregate to the interfaces during operation, leading to the formation of t-YSZ in the Y-depleted regions. (C) 2011 Elsevier B.V. All rights reserved.