Low-pressure diamond deposition from hydrocarbon/hydrogen ps-phase mixtures was discovered over 20 years ago, but the exact deposition mechanism remained unknown. Work reported here confirms preferential graphite erosion to be the rate-controlling step, as had been assumed by some researchers earlier, and identifies reaction pathways by which the graphitic constituents of the deposit are preferentially removed. Model systems of C:H a few monolayers thick were used to show that dehydrogenation, erosion, and hydrogenation all play a part.