SOFCs were fabricated employing a single step co-firing process and electrochemically characterized in the temperature range of 700-800oC with humidified hydrogen (3% H2O) and air. Voltage-current density characteristics (V-i) at 800oC in hydrogen-water vapor mixtures with compositions ranging from 3% to 73% water vapor have been obtained to simulate the effect of practical fuel utilization on the performance of single cells. The V-i plots obtained in hydrogen containing 3% water vapor were curve-fittd with a model to obtain the ohmic resistance of the cell, the exchange current density and the cathodic and anodic limiting current densities. The V-i curves obtained in other fuel compositions were successfully modeled by varying only the exchange current density and a physical interpretation of the anodic reaction was proposed.