A first principles model for predicting acid leaching phenomenon in high temperature proton exchange membrane fuel cells (HTPEMFC) containing phosphoric acid as the electrolyte has been developed. A 1-D dynamic HTPEMFC model was adopted from literature and a mechanism is proposed for the acid leaching process in PBI membrane, which was integrated into the dynamic model. The dilution of phosphoric acid in HTPEMFC membrane has been assumed to occur, leading to a drop in viscosity of the proton conducting electrolyte. The stiction holding the acid in the pores of the membrane varies with the degree of dilution. The relationships between surface tension, viscosity, temperature, H₃PO₄ weight percentage were obtained from data. The limiting stiction and viscosity beyond which acid leaching becomes inevitable were simulated, and the electrolyte viscosity was found to approach these limiting values over time. The proton conductivity of HTPEMFC membrane was found to decrease with acid leaching, which eventually leads to the failure of the membranes.