Abstract It has been known for some time that the sun emits radio-frequency radiation whose intensity greatly exceeds the value expected from a black-body at 6000°K. In the present paper, experiments are described in which measurements have been made of the solar radiation at frequencies of 175 and 80 Mcyc. /sec. Measurement of the small powers which can be abstracted from practical aerial systems requires special types of receiving equipment if absolute measurements are to be recorded automatically over long periods of time. An apparatus has been developed in which the output power of a local source of random ‘noise’ is automatically and continuously adjusted so as to be equal to the aerial power; in this way the receiver is used only as an indicator of balance, and errors due to variation of its gain or internal noise are eliminated. A special type of aerial has been devised which enables the solar radiation to be recorded separately from the galactic radiation, and so enables continuous observation of the sun to be made with aerials of comparatively low directivity. The results obtained on these two frequencies show that the sun normally emits radiation whose intensity corresponds to a surface temperature of the order of 106°K. Large fluctuations in the intensity occur, however, and during the passage of large sunspots, equivalent temperatures as high as 108 to 109°K have been observed. In addition to these day-to-day variations the radiation is subject to sudden brief increases of intensity lasting only for a few seconds. Measurements of the diameter of the source, by a method analogous to Michelson’s stellar interferometer, have shown that during periods of very great intensity the radiation originates in an area of the sun of the same order of size as a sunspot. This result means that equivalent temperatures of 109 to 1010°K must exist. Measurements of the polarization of the radiation have shown that during periods of increased activity the radiation is mainly circularly polarized. The present account covers the experimental methods and the results obtained up to the present time. It is hoped to consider these results theoretically in a future paper.