The Li/oxygen battery may achieve a high practical specific energy as its theoretical specific energy is 11,400 Wh/kg Li assuming Li2O2 is the product. To help understand the physics of the Li/oxygen battery we present the first physics-based model that incorporates the major thermodynamic, transport, and kinetic processes. We obtain a good match between porous-electrode experiments and simulations by using an empirical fit to the resistance of the discharge products (which include carbonates and oxides when using carbonate solvents) as a function of thickness that is obtained from flat-electrode experiments. The experiments and model indicate that the discharge products are electronically resistive, limiting their thickness to tens of nanometers and their volume fraction in one of our discharged porous electrodes to a few percent. Flat-electrode experiments, where pore clogging is impossible, show passivation similar to porous-electrode experiments and allow us to conclude that electrical passivation is the dominant capacity-limiting mechanism in our cells. Although in carbonate solvents Li2O2 is not the dominant discharge product, we argue that the implications of this model, (i.e., electrical passivation by the discharge products limits the capacity) also apply if Li2O2 is the discharge product, as it is an intrinsic electronic insulator. (C) 2011 The Electrochemical Society. [DOI:10.1149/1.3527055] All rights reserved.