Porous catalyst layers containing sulfonated polyetheretherketone (sPEEK, 1.4 meq g(-1)) were deposited onto membranes to form catalyst-coated membranes. The catalyst layer properties, as a function of sPEEK content, were studied ex situ and in fuel cells. Particle sizes observed for sPEEK-catalyst inks were smaller than for corresponding Nafion-catalyst inks. Consequently, particle sizes in spray-coated catalyst layers were also smaller. Catalyst layers containing sPEEK were observed to densify with increasing ionomer content in contrast to Nafion-catalyst layers. The catalyst layer thickness decreased from 14 to 4 mu m when the sPEEK content was increased from 10 to 40 wt %. Pore volumes also decreased and were lower than Nafion-catalyst layers. The optimum sPEEK content of the cathode layer for membrane electrode assemblies was 20 wt %. Lower sPEEK contents compromised proton conductivity, whereas higher sPEEK contents led to excessive flooding of the cathode. Relative humidity of the gases exerted a strong influence on the cathode and fuel cell performance. Too much hydration resulted in excessive flooding; too little led to a significant loss in proton conductivity. Although this paradox is typical of proton exchange membrane fuel cells, it is exacerbated by sPEEK for which water sorption and proton conductivity are highly sensitive to external conditions. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3454735] All rights reserved.