In this work, gas phase processes in a high pressure (~100 Torr) DC hydrocarbon plasma are investigated in situ by optical emission spectroscopy and mass spectroscopy. In the high pressure plasma, optical emission characteristics of glow layers are obviously different. C2 , CH dominated band spectra and discrete spectra are distinctively observed in the positive column, whereas the emission intensity is found to decrease in the anode region. In the cathode region, a large number of complicated spectra are d...