H-2, CO electro-oxidation in a model PEM Fuel Cell (PEMFC) is investigated at 37 degrees C with the help of distributed current measurements. The main purpose of the experiments is to verify the formation of spatio-temporal patterns that have been predicted for a PEMFC with straight channels and a H-2 evolving cathode. For galvanostatic operation and a high feed flow rate well expressed globally coupled oscillations are found. For decreasing feed flow rate a period doubling bifurcation cascade appears. Potentiostatic control provokes the breakage of the strict phase relation between adjacent oscillators and the appearance of pulses as well as turbulence is seen. The turbulence is enhanced if an oxygen reducing cathode is introduced instead of a hydrogen evolving cathode. All the findings are compared to existing predictions. The study of the patterns is of interest for PEMFC engineers as they directly determine the CO tolerance behavior of the PEMFC. (C) 2013 The Electrochemical Society. [DOI: 10.1149/2.002306jes] All rights reserved.