Optical emission spectroscopy, Langmuir probes, and calorimetric techniques have been used to characterize the plasma jet in a DC plasma torch diamond deposition apparatus. The experimental data are compared to calculations based on a thermodynamic equilibrium model. The ion temperature obtained from the atomic hydrogen emission is within 10% of the calorimetrically deduced gas temperature, while the electron temperature is approximately a factor of two higher. For a wide range of process conditions, the plasma temperature is almost independent of the torch input power, but depends strongly on the hydrogen concentration in the feed gas. Calorimetry provides a means of measuring the concentration of atomic hydrogen produced by the torch.