Liquid leak detection systems, and fuel cell systems and hydrogen-producing fuel processing systems containing the same. The liquid leak detection systems are adapted to detect liquid leaks from the fuel cell and/or fuel processing systems and to respond thereto, such as by interrupting the delivery of liquid reactants, by generating an alert, by transitioning the fuel processing system to a different operating state, and/or by shutting down the fuel processing and/or fuel cell system. The liquid leak detection systems may include a controller and at least one liquid detector, and in some embodiments may include a cover. In some embodiments, the liquid detector includes a signal emitter and a signal detector. In some embodiments, the liquid detector includes at least a pair of spaced-apart conductive members that nominally define an open circuit. In some embodiments, the cover is adapted to repel water and/or includes conductive particulate.