We present a model of transport and reaction kinetics in ultrathin cathode catalyst layers for polymer electrolyte fuel cells (PEFCs). In contrast to conventional catalyst layers, ultrathin catalyst layers neither contain a separate electronic conductor, other than the catalyst itself, nor are they impregnated with perfluorinated sulfonic acid ionomer as an intrinsic proton conductor. They can thus be regarded as two-phase composites. The model utilizes Poisson-Nernst-Planck theory for proton transport in the layer. It relates calculated spatial distributions of oxygen and proton concentrations, electrode potential, and electrochemical reaction rates to catalyst utilization and current-voltage performance. By comparison with experimental data from literature for current-voltage relations at low and intermediate current densities the transfer coefficient alpha(c) of the cathodic reaction was estimated. Catalyst layer thickness, composition, and volume fraction of water-filled pore were systematically varied to determine the values that maximize Pt utilization and voltage efficiency of the layer. The significance of these results for the optimization of catalyst layers in view of operation conditions and synthesis methods is discussed. (c) 2007 The Electrochemical Society.