The effect of trace amounts of formic acid (HCOOH) in hydrogen fuel on proton exchange membrane fuel cell (PEMFC) performance is reported. Long-term stability tests (100 h), periodic cyclic voltammetry scans, and electrochemical impedance spectroscopy analyses are used to evaluate and characterize the effects of this impurity on fuel cell performance. The results show that trace amounts of HCOOH cause degradation in fuel cell performance and significantly contaminate the electrodes. Furthermore, full recovery from the contamination could not be achieved by applying pure hydrogen to the anode while operating the fuel cell. However, this degradation may also be caused by the coarsening or dissolution of Pt, in addition to any permanent effects of HCOOH contamination. Mechanisms of contamination of the electrodes and performance degradation of the PEMFC are also postulated.