Methods and results of measurements of the tensor conductivity and electron temperatures in argon-potassium and helium-potassium plasmas with elevated electron temperatures under MHD generator conditions are described. Electrode and wall effects that would make the measurements more difficult to interpret have been eliminated. Calculations of W. Feneberg predict that the scalar electrical conductivity should drop slightly with increasing magnetic field strength. However, the measured effective scalar conductivity was less than the theoretically predicted value. The discrepancy is apparently due to instabilities. These instabilities were investigated. At current densities and field strength values at which instabilities were observed, the effective Hall coefficient has been found to be nearly independent of the magnetic field strength. The measurements have shown that the efficiency of a noble gas MHD generator is adversely affected not only by the decrease of the scalar conductivity in a magnetic field, which has been predicted by W. Feneberg, but also by the onset of instabilities. © 1968 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.