Fuel cells are getting growing interest in both backup systems and electric vehicles. Although these systems are characterized by periods of standby, they must be able to start at any instant in the shortest possible time. However, the membranes of which proton exchange membrane fuel cells are made, tend to gradually dry out when the fuel cell is not operating, increasing the time required to start up the system. A precise estimation of the hydration status of the membrane during standby is thus important for the design of a fuel cell system capable of a fast and safe start up. In previous works, the measurement of the complex impedance of a fuel cell stack during standby is used as an index of its membrane hydration status. In this article, the complex impedance of a fuel cell stack has been measured and characterized as a function of relative humidity and temperature. A non-conventional electrochemical impedance spectroscopy (EIS) technique has been used, allowing the performance of a fuel cell diagnostic when the fuel cell stack does not contain any hydrogen, which would otherwise not be possible. The results appeared to confirm that measuring the impedance of an entire fuel cell stack could be a viable way for estimating the hydration status and the temperature of its membrane before the system is started up. A summarizing table with the complete characterization of the fuel cell stack is included in this article.