A calorimetric technique was used to measure the plasma enthalpy in a d.c. arcjet diamond deposition system. Using these measurements, and a model based on the assumption of local thermal equilibrium (LTE), the temperature of the plasma emerging from the torch nozzle could be calculated. By controlling the electrical power into the torch, the plasma temperature could be varied from 2900 to 4500 K. This range of plasma temperatures corresponded to a fraction of dissociated H-2 (alpha) which ranged from 0.19 to 0.98. Surprisingly, this variation in the concentration of atomic hydrogen had no effect on the diamond growth rate.