Fuel processing systems that are used in conjunction with polymer electrolyte membrane (PEM) fuel cells require high activity water gas shift (WGS) catalysts that remove carbon monoxide, a poison for membranes in the fuel cell stacks. WGS activity of various noble metals supported on cubic, nanocrystalline, high surface area, large pore ceriazirconia was measured at a space velocity of 312,000 hr(-1), under two feed gas compositions that simulated the high temperature shift (HTS) and the low temperature shift (LTS) conditions found in a typical fuel processing system. The (HTS) feed composition was 4.9% CO, 10.5% CO2, 33% H2O, 30.3% H-2; and the low temperature gas composition (LTS) was 1.5% CO, 5% CO2, 45% H2O, 25% H-2, with the balance of the stream being N-2 in both cases. The activity trend under both feed gases over a broad range of temperatures (200 degrees C-320 degrees C) was Pt > Rh > Ru similar to Pd > Ir > Au. This trend did not agree with those previously reported in literature or with CO uptake data for these samples. We hypothesize that this reflects the dominance CO different reaction mechanisms under different gas compositions and temperatures. The discrepancy in the activities observed in this study and those reported in literature might also be attributed to differences in the synthesis techniques of the oxide supports and metal loading. Our study also found that the turnover rate (TOR) for the Pt catalyst was maximized in the range between 1-2 wt% loading under both HTS and LTS conditions. (c) 2006 American Institute of Chemical Engineers.