Polymer electrolyte membrane fuel cells (PEMFCs) require gaskets and seals in each cell to keep the reactant gas and liquid within their respective regions. The durability of sealing materials is crucial to the operating life as well as the electrochemical performance of fuel cell stacks. In this study, the aging process of silicone rubbers, often used as gasket materials for fuel cells, with hardnesses of 30, 40, and 50 (Shore A) in a solution collected from the cathode of PEMFC is investigated. Fourier transform infrared spectroscopy and an inductively coupled plasma atomic emission spectrometer are used to study the chemical composition of the samples before and after exposure to the test environment over time. It is suggested that the degradation of silicone rubbers is attributed to the chemical decomposition of silicon-based backbone and the leaching of fillers. Although the silicone rubber with higher hardness loses more weight due to its larger amount of fillers, it seems more stable and durable in mechanical properties and surface morphology.