In this work, the reforming activity of Ni-Ce0.75Zr0.25O2 (Ni-CZO) solid oxide fuel cell (SOFC) cermet anodes for steam reforming of methane has been investigated in the absence of electrochemical effects and the results were compared with Ni-Ce0.9Gd0.1O2 (Ni-CGO) cermet anodes. From X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy/energy dispersive X-ray spectroscopy (TEM/EDS) and CHN analysis on the spent Ni-CZO anode cermets, it was found that a decrease in reforming activity at higher steam-to-carbon ratios (S/C) was due to sintering of nickel nano-particles rather than re-oxidation or sintering of bulk nickel and carbon formation. Time-on-stream analysis demonstrated a higher reforming activity for Ni-CZO cermet anodes compared to Ni-CGO cermet anodes. While the initial sizes of nickel nano-particles were similar (5-10 nm) for both cermet anodes, after the operation under steam-rich conditions (S/C = 1.5) the nickel nano-particles on Ni-CGO were found to be significantly larger (20-40 nm) than those on Ni-CZO (15-25 nm). Under steam-lean conditions (S/C = 0.5), the size of nickel nano-particles remained below 10 nm for both cermet anodes during the operations, indicating that the effect of sintering of Ni nano-particles on the performance was negligible. The reforming activity is still higher for Ni-CZO cermet anodes than Ni-CGO cermet anodes, which could be attributed to higher concentration of ex-solved nickel nano-particles as well as lower Ce4+/Ce3+. reduction energy of CZO than CGO support. (C) 2011 Elsevier B.V. All rights reserved.