Two diamond combustion CVD systems have been investigated with respect to the reaction mechanism in the gas phase: an atmospheric acetylene-oxygen welding torch and a flat, premixed acetylene-oxygen-argon flame at 50 mbar. The atmospheric pressure flame was used to study the influence of important deposition parameters including stoichiometry, deposition time, substrate temperature and pretreatment as well as that of some chemical additives. In the low-pressure flame, the chemical composition in the gas phase was investigated in detail for some specific deposition conditions. A combination of laser techniques and molecular beam mass spectrometry was used to measure spatially-resolved temperature and species concentrations with and without a substrate present. The results for some stable and intermediate species are compared with recent numerical models of the gas phase chemistry. An overview is given of some recent experiments, and perspectives for further work an discussed.