A powerfull P. I. G. reflex arc in lithium vapour has been studied using spectrographic, mass spectrometric and probe measuring techniques. The Li-vapour has been produced in a special Li-oven; the vapour entered through a small tube directly into the hollow anode cylinder. The residual gas pressure was 10-7 mm Hg with the arc in operation, the density in the plasma column was about 1013 cm-3. The discharge showed two different modes of operation. We observed that at a fixed flow of Li-vapour the discharge can be operated in a stable state (Mode I) only below a certain limit of the applied electrical power. An increase of electrical power (or a decrease of the flow of Li-vapour) causes a strong instability (Mode II). 1. Mode I showed the discharge column to be very well stabilized by the confining magnetic field. Electrostatic probe measurements showed no voltage fluctuations. The electron temperature ranged from 1.8 to 8 eV, depending on the electrical power applied. 2. Mode II showed the plasma column to become diffuse. Large voltage fluctuations of several volts in the megacycle region could be detected on the electrostatic probe. A powerfull flux of ions leaving the plasma column perpendicular to the magnetic lines of force and hitting the walls with energies up to 327 eV has been measured. These energetic ions are responsible for the strong light emission of the walls of the vacuum tube, this light intensity being much higher than the intensity of the plasma column. The experimental results are similar to those observed by NEIDIGH, WEAVER, NEZLIN and SOLNTSEV under different experimental conditions.