The possibility of reducing low-boiling metal halides to the metal in thermal plasmas is attracting increasing attention. However, instability of the arcs in the presence of even low halide concentrations has so far thwarted all research efforts. An experimental study of the effects of adding TiCl4 to an argon transferred arc has shown that instabilities and eventual extinction of the arc are due to severe chemical corrosion of the thoriated tungsten cathode in the presence of chloride. The corrosion results in both the loss of cathode material and the deposition of a blanket of titanium metal on the cathode's surface which supresses electron emission. A systematic search has shown that tantalum carbide cathodes will provide stable operation. Additions of TiCI4 cause a sharp increase in total arc voltage, largely due to an increased cathode /all potential.