Although some researchers have shown the possibility of using PEMFC sensors with solid polymer electrolyte to measure ethanol gas concentration [21,22,38], no study has been done so far to show the effect of catalyst loading on the performance of these sensors and how much the catalyst loading in commercial sensors can be decreased if their fabrication technology is updated. The objective of this study is to answer these questions. For this purpose, the solid polymer electrolyte membrane (Nafion) and electrodes containing carbon-supported Pt catalyst were employed to fabricate the PEMFC sensors. These sensors were used to measure ethanol gas concentration in the simulated exhaled human breath. Electrodes with different Pt catalyst loading were examined to investigate the effect of Pt loading on performance of fresh sensors. The fresh sensor in this study denotes a sensor tested in the same day that the membrane electrode assembly (MEA) has been fabricated. It is noted that in addition to the catalyst loading [39] that is the purpose of this study, the type of catalyst [40] and the electrode microstructure [41–45] can play important roles in performance determination of any electrochemical systems, including fuel cell sensors. The type of catalyst, the electrode microstructure, durability and environmental tests, and improvement of polymer electrolytes for fuel cell sensors are ongoing studies in Advanced Energy & Sensor Lab.