A negative effect on the quality of diamond film because of the addition of hydrogen to carbon‐dioxide–hydrocarbon gases was investigated. To elucidate the role of hydrogen and oxygen in diamond synthesis using microwave plasma chemical‐vapor deposition, diamond films were deposited by adding hydrogen and oxygen to carbon‐dioxide–methane‐gas mixtures. Improvements in the quality of diamond film and extensions of the diamond‐forming region were obtained, due to the addition of oxygen to the carbon‐dioxide–methane‐gas mixtures. The nucleation density of deposits increased when the amount of hydrogen was increased but decreased when the amount of oxygen was increased. The results of optical emission spectroscopy indicate that the amount of atomic hydrogen in the ground state remained nearly the same, respectively, with increasing amounts of hydrogen and oxygen. However, the amount of electronically excited atomic hydrogen and C2emissions increased with an increase in the amount of added hydrogen; this means that the electronically excited atomic hydrogen did not benefit diamond growth and the C2radicals facilitated the formation of amorphous or graphitic carbon. In contrast to the addition of hydrogen, when oxygen was added, the electronically excited atomic hydrogen and C2radicals decreased, and a larger amount of oxygen‐containing species such as atomic oxygen and OH radicals was in the plasma, so the quality of the diamond film improved. In summary, adding hydrogen to the carbon‐dioxide–methane‐gas mixtures had a negative effect on diamond growth, whereas adding oxygen had a beneficial effect.