PEM (proton exchange membrane) fuel cell is a device that converts chemical energy stored in fuel (hydrogen) into electrical energy via electrochemical reaction with oxygen. Even though fuel cell have high efficiency when pure hydrogen from gas tank is used as a fuel source, it is more beneficial to generate hydrogen from city gas (mainly methane) in residential application such as domestic or office environments. Thus hydrogen is generated by reforming process using hydrocarbon. Unfortunately, during a fuel reforming process, additional species such as CO, CO₂, H₂S, NH₃, and CH₄ may also be produced. PEM fuel cell has problems with catalyst poisoning by impurities in hydrogen fuel. The aim of the research is to investigate cell performance decline due to carbon monoxide impurity in hydrogen. Performance of PEM fuel cell was investigated using current vs. potential experiment, long run (10 hours) test and electrochemical measurement (impedance, cyclic voltammetry). The concentrations of carbon monoxide were changed up to 10 ppm. Performance degradation due to carbon monoxide contamination in anode fuel was observed at high concentration of carbon monoxide. The performance recovery was confirmed in long run test when pure hydrogen was provided for 1 hour after carbon monoxide had been supplied. The result of this study could be used as basis of various reformation processes design and fuel quality determination.