Thin-film GDC (Ce0.8Gd0.2O2-x 300 nm thick) electrodes with porous Ni or Au metal overlayers (300 nm thick) were sputter-deposited on polycrystalline YSZ substrates and tested in single-chamber cells. With a back-side Pt-counter electrode, the electrochemical cells were characterized by linear-sweep voltammetry and impedance spectroscopy between 650 to 750 ºC in mixtures of H2/H2O/Ar diluent. To complement the electro-chemical tests, ambient-pressure XPS at the Advanced Light Source was performed on active cells in H2/H2O mixtures to measure surface potential and oxidation states. The GDC electrodes showed lower resistance to H2O electrolysis than H2 oxidation with Ni and Au overlayers. XPS measurements indicated large overpotentials across the YSZ/GDC interface particularly for H2 oxidation on the GDC electrode. Increasing PH2/PH2O (up to 10) resulted in higher rates for not only H2 oxidation, but also H2O electrolysis, suggesting that H2O electrolysis activity is enhanced by increased reduction of the GDC surface.