Microwave discharges of H2 admixed with CH4 in a moderate-pressure quartz bell jar reactor used for diamond deposition are studied numerically. Special attention was devoted to high-power densities which provide the most effective way for producing high-quality diamond films. First, a one-dimensional radial model describing the coupled phenomena of chemistry, energy transfer, as well as species and energy transport along the reactor’s radial coordinate was developed. Species densities predicted with the model were compared with measurements with infrared tunable diode laser spectroscopy, resulting in validation of the model. Second, a one-dimensional axial model was used to describe the plasma flow along the reactor axis in a region between the reactor end wall and the substrate surface. This model was particularly useful for studying the plasma behavior in the vicinity of the substrate surface, where thermal and composition gradients are large. Both the radial and axial transport models are based on the same discharge model in which the plasma is described as a thermochemically nonequilibrium flow with different energy distributions for heavy species and electrons. The chemistry was described with a model containing 28 species and 131 reactions. The electron temperature, the gas temperature, and the species concentration were determined by solving a coupled set of equations. A wide range of experimental conditions used for diamond deposition was simulated, from low microwave power density (9 W cm-3, i.e., 600 W, 2500 Pa, and Tg∼2200 K) to high-power density (30 W cm-3, i.e., 2 kW, 12 000 Pa, and Tg∼3200 K). The main chemical paths were identified, and the major species, transport effects, and reaction pathways that govern diamond deposition plasmas are discussed.