A segmented electrode/current collector setup was used to examine the effect of oxygen flow rate, anode sparger temperature, and hydrogen starvation on the spatial and temporal distribution of local current densities along a single gas channel in a PEM fuel cell operated on pure hydrogen and oxygen. Uniformity in local current densities at a given voltage was sustained over longer periods of time with higher oxygen flow rates that,9 enhanced liquid water removal. When water removal rate was not sufficient, electrode flooding occurred in segments that were farthest from the gas inlet. Increasing anode sparger temperature resulted in reduction of water removal rate from the cathode by back diffusion leading to performance reduction of downstream segments. When fuel was in excess, the current density distribution was dominated by the hydration pattern of the membrane electrode assembly. When fuel supply was less than the electrochemical reaction demand, segments furthest from the inlet suffered fuel starvation, resulting in loss of performance. The use of one of the electrode segments as a novel in-line voltage sensor to reflect available reactant concentrations was also demonstrated. Air operation and effect of cell temperature were also examined and the results are presented ill the second part of this series. (c) 2005 American Institute of Chemical Engineers.