The durability of proton exchange membranes (PEMs) is a critical factor in the future commercial success of hydrogen fuel cell technology. Presently, the favored PEM material is made of perfluorinated suffonic acid (PFSA) ionomers. Current efforts to improve PFSA durability are hindered by the lack of thorough understanding of the chemical mechanism by which PFSA degrades. In this paper, the PFSA chemical degradation mechanism is reviewed which incorporates two possibilities for initiation along the PFSA polymer: weak polymer end groups versus side chain cleavage. A kinetic model is developed which quantitatively distinguishes between these two degradation initiation mechanisms. Experimentally, degraded PFSA samples were obtained under both fuel cell and ex situ Fenton's test conditions, and infrared spectroscopy (IR) was used to measure the relative concentration of carboxylic acid end groups on the degraded polymer chains. The IR data, coupled with the corresponding ionomer fluoride loss data, validate the kinetic model and allow calculation of the kinetic constants which distinguish between the two degradation initiation mechanisms. Theoretical case studies are also included to illustrate the usefulness of the kinetic model. (C) 2007 Elsevier Ltd. All rights reserved.