Diamond deposition in a flat, premixed acetylene-oxygen-argon flame at 50 mbar was investigated to characterize the reactive gas phase in the vicinity of the substrate. For this, flames with and without a substrate present were analyzed as a function of stoichiometry; also, the distance between the substrate and the burner was varied. Optimal conditions for the deposition of diamond films were found for oxygen-acetylene ratios of 1.3 and 1.4 and at distances between substrate and burner of 8, 9, and 10 mm. The flame structure in this region was investigated. In particular, gas temperature and OH radical concentrations were measured by laser-induced fluorescence (LIF). Furthermore, hydrogen atoms were monitored using three-photon excitation and subsequent fluorescence detection. Molecular beam mass spectrometry was employed to obtain an overview of stable species and hydrocarbon intermediates. The results provide a substantial experimental basis for comparison with theoretical models and are consistent with earlier observations, which stress the importance of H and CH3 for the diamond deposition process. In addition, the observations indicate the participation of hydrocarbon species with more than 2 carbon atoms, e.g., C3H3, C4H3, and CxH2 with x = 4, 6, or 8, in the gas-phase reactions controlling the deposition of diamond; an active role for these species in diamond chemical vapor deposition (CVD) has not been discussed before. As a first interpretation, diamond formation seems to be controlled by a counterbalance between OH and hydrocarbon intermediates at a position in the flame where sufficient H-atoms and CH3 radicals are present to support diamond film growth.