The response of Gore's advanced PRIMEA(R) Series 5561 membrane electrode assembly (MEA) exposed to transient concentrations of CO in the anode feed was studied for a 25 cm(2) laboratory-scale polymer electrolyte membrane fuel cell (PEMFC). The data include relatively high (500 and 3000 ppm) CO levels at 70 degreesC cell temperature, low reactant stoichiometry, and atmospheric pressure, conditions that may be typical for stationary PEMFC applications. Poisoning and recovery rates are reported for saturated conditions and these rates are compared for two types of gas diffusion media [single-sided ELAT(R) and CARBEL(TM) CL gas diffusion media (GDM)] and for conditions with and without air-bleed treatments, It is shown that a 5% air bleed provides a current density of 1.0 A/cm(2) at 0.6 V for CARBEL CL GDM exposed to 500 ppm CO/H-2 mixtures. The data show that the transient performance at 0.6 A/cm(2) with this MEA and relatively high concentrations of CO is a result of an interaction of CO kinetics and mass transfer through the GDM. Indirect evidence of electrochemical oxidation of CO during the transient pulses with 3000 ppm CO is presented. The data discussed in this paper are suitable for verifying numerical models of a PEMFC and establishing a baseline for new recovery schemes using new MEAs with enhanced CO tolerance. In addition, the results have implications for the design of reformate fuel processing systems and the use of effective control schemes to prevent CO transients.