The chemistry of hydrogen-rich hydrocarbon-hydrogen mixtures is of primary interest for tire understanding of the low-pressure synthesis of diamond. We performed experiments under well-defined conditions like temperature, pressure, initial gas composition, etc. The gas composition at the end of a flow reactor was analyzed by a calibrated mass spectrometer and compared to results obtained from the Chemkin computer code. Residence time in the reactor as well as other process parameters sere similar to those of diamond-growing PACVD processes performed earlier with the same experimental set-up. Modeling and experiment under isothermal conditions show quantitative agreement. We realized time-resolved mass spectrometry by means of a helium-flushed gas sampling probe. There is evidence that the commonly used reaction kinetic data for tire dissociation C2H6 (+M) double left right arrow 2CH(3)(+M) gives too small C2H4 concentrations for hydrogen-rich conditions. This could be attributed to the poorly known third-body efficiencies of the H-2 molecules compared to Ar or C2H6 from which kinetic data are commonly derived.