Negatively charged clusters of 3000-18 000 atomic mass units, which had been predicted by the charged cluster model, were experimentally confirmed under typical process conditions of hot-filament diamond CVD using gas mixtures of 1-5% CH4 and H-2. The cluster size increased with increasing methane concentration. Under conditions for the generation of small clusters containing a few hundred carbon atoms, high-quality diamond films were deposited while under conditions for the generation of larger clusters (> similar to 1000 atoms), a cauliflower structure was obtained.