A model of cathode catalyst layer (CCL) performance by Perry [J. Electrochem. Soc., 145, 5 (1998)] shows that there exists optimal effective diffusion coefficient of oxygen D-*, which provides close to minimal voltage loss eta(0) in the CCL. The dependence eta(0)(D) has two slopes: at D < D-*, eta(0) decreases rapidly, whereas for D>D-*, lowering of eta(0) due to better oxygen transport is marginal. The value of D-* is proportional to the product of cell current and CCL thickness. Oxygen diffusivity depends on CCL porosity; thus, D-* may serve as a reference point for optimization of CCL porosity and related Nafion content.