The microwave and hard-X-ray burst of 1970 March 01, 11:27 UT was found to originate from a common thermal plasma with a maximum temperature of 57 keV. The low coronal plasma with an average electron density of about 3.108cm−3 covered a projected area of 5.1018 cm2. In Fig. 1 the time profiles of the emission measure and the temperature are compared with the 10.5 GHz flux while Fig. 2 shows the reversible relationship between the hard X-ray emission measure and temperature during the impulsive phase. The arrows indicate the direction of increasing time. The dashed-dotted line, representing an adiabatic process with an index χ = 5/3, agrees well with the observations showing a compression followed by an expansion (Mätzler et al. 1978).