In this paper $\mathrm{Ar}∕{\mathrm{H}}_2∕\mathrm{C}{\mathrm{H}}_4$ microwave discharges used for nanocrystalline diamond chemical vapor deposition in a bell-jar cavity reactor were characterized by both experimental and modeling investigations. Discharges containing 1% $\mathrm{C}{\mathrm{H}}_4$ and ${\mathrm{H}}_2$ percentages ranging between 2% and 7% were analyzed as a function of the input microwave power under a pressure of $200\mathrm{mbar}$ . Emission spectroscopy and broadband absorption spectroscopy were carried out in the UV-visible spectral range in order to estimate the gas temperature and the ${\mathrm{C}}_2$ density within the plasma. Infrared tunable diode laser absorption spectroscopy was achieved in order to measure the mole fractions of carbon-containing species such as $\mathrm{C}{\mathrm{H}}_4$ , ${\mathrm{C}}_2{\mathrm{H}}_2$ , and ${\mathrm{C}}_2{\mathrm{H}}_6$ . A thermochemical model was developed and used in order to estimate the discharge composition, the gas temperature, and the average electron energy in the frame of a quasihomogeneous plasma assumption. Experiments and calculations yielded consistent results with respect to plasma temperature and composition. A relatively high gas temperature ranging between 3000 and $4000\mathrm{K}$ is found for the investigated discharge conditions. The ${\mathrm{C}}_2$ density estimated from both experiments and modeling are quite high compared with what is generally reported in the literature for the same kind of plasma system. It ranges between ${10}^{13}$ and ${10}^{14}{\mathrm{cm}}^{-3}$ in the investigated power range. Infrared absorption measurements and model predictions indicate quite low densities of methane and acetylene, while the atomic carbon density calculated by the model ranges between ${10}^{13}$ and ${10}^{15}{\mathrm{cm}}^{-3}$ . The methane and hydrogen introduced in the feed gas are subject to a strong dissociation, which results in a surprisingly high H-atom population with mole fraction ranging between 0.04 and 0.16. Result analysis shows that the power coupling efficiency would range between 70% and 90%, which may at least explain the relatively high values obtained, as compared with those reported in the literature for similar discharges, for gas temperature and ${\mathrm{C}}_2$ population. The high H-atom densities obtained in this work would indicate that growing nanocrystalline diamond films would experience a very high etching. Simulation results also confirm that sp species would play a key role in the surface chemistry that governs the diamond growth.