Plasma parameters were determined for a direct current arcjet diamond chemical vapor deposition reactor. Electron temperatures and densities were measured using a floating double probe technique, yielding values ranging from 2.3 eV and 7×1011 cm−3 near the plasma gun nozzle to 0.4 eV and 1×109 cm−3 in the downstream and peripheral regions of the plasma. These results show that the plasma is weakly ionized, indicating that reactions involving electrons play a minor role in the macroscopic gas–phase chemistry though high energy products from these reactions may significantly affect film quality.