A novel reactor has been constructed to investigate the fundamental surface kinetics of diamond chemical vapor deposition(CVD). The molecular beam reactor permits independent control of the atomic hydrogen flux, the methyl radical flux and the substrate temperature. The low pressure in the growth chamber (≈ 1 mTorr) minimizes the impact of gas-phase chemistry. The reactive mixture impinging the substrate is sampled through an orifice and analyzed by mass spectroscopy. Differential pumping in the two adjacent chambers quenches the beam, allowing quantitative analysis of radical species such as H and CH3. In preliminary experiments deposition was achieved onto seeded molybdenum substrates.