Three batches of SPEEK based PEMs were modified by embedding a free radical scavenger, catechin, belonging to a group of polyphenolic compounds, which is known for its biological activity and exhibits a high scavenging effect. Initial and modified membranes were characterized by a number of physicochemical methods. It was found that blending with catechin strongly affects membrane properties, reducing their swelling and decreasing their proton conductivity. The performance of the blended PEMs tested in hydrogen/air FC slightly but consistently declined with catechin loading. Thermo-gravimetric experiment and Fenton's test have shown a loss of thermal and chemical stability of the membranes when modified with radical scavenger. The experimental results are rationalized on the basis of the assumption that the decline of PEMs functional properties is associated with scavenger specific behaviour in the acidic environment of the polymer. Contrary to the biological systems, the scavenger incorporated in sulfonated polymer bulk, undergoes chemical interaction with pendant acid groups and apart from scavenging effect shows an important radical-generating property, particularly strong during thermolysis or in Fenton's test. This makes blended membranes even more vulnerable to the FC working environment. A conclusion was made that mixing with a free radical scavenger is not a promising approach to enhance the chemical stability of hydrocarbon based PEMs intended for FC application. (C) 2009 Elsevier B.V. All rights reserved.