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Fuel cells have started replacing traditional lead battery banks in backup systems. Although these systems are characterized by long periods of standby, they must be able to start at any instant in the shortest time. In the case of low temperature proton exchange membrane fuel cell systems, a precise estimation of the fuel cells's membrane hydration status during standby is important for a fast and safe startup. In this article, the measurement of the complex impedance of the fuel cell is suggested as a method to estimate the membrane hydration status. A Nation-based membrane electrode assembly has been fed with air whose temperature and relative humidity were controlled, and its complex impedance has been measured at different frequencies and for different values of the relative humidity. After characterizing the cell, a first approach in finding the relationship between relative humidity and complex impedance using exponential regression functions has been suggested. The results showed that measuring the impedance of a fuel cell can be a viable way for estimating the hydration status of its membrane before the system is started up.