Quantitative data of the effect of contaminants on individual components of a PEMFC is limited and difficult to acquire, especially for the ionomer in the catalyst layer. In this paper, we propose the use of an acid-catalysed reaction (esterification) as a method to quantitatively investigate the effect of contaminants on proton availability and conductivity of Nafion components, since proton sites in Nafion are also active as Bronsted acid sites for catalysis. It was found that at typical fuel cell conditions, ammonia adsorption decreased both conductivity and esterification activity of Nafion in a uniform manner. Because of the linear relationship between the number of proton/acid sites and both the conductivity and the esterification activity, a correlation between the two could be developed taking into account differences in the effect of humidity on the conductivity/activity of the poisoned Nafion. The methodology and correlation developed were also shown to predict accurately the effect of another impurity species (Na+) on Nafion conductivity. The results demonstrate the application of esterification as a means to quantify the number of proton sites poisoned by adsorbing impurities, permitting the prediction of Nafion conductivity. This method would be applicable to both the membrane and ionomer in the catalyst layer. (C) 2009 Elsevier B.V. All rights reserved.