Direct measurements of the ratio of the surface absorptivity of sunlight to the thermal emissivity have been made, and the apparatus is described. A thin plate to be measured is suspended in a vacuum, surrounded by walls cooled by liquid nitrogen, and illuminated by an arc light, which simulates solar radiation. The desired ratio is determined from the equilibrium temperature, which ranged from 0° to 250°C for the samples measured. Possible errors in the procedure have been analyzed theoretically, and corrections introduced where necessary. Intensity fluctuations in the present light source have limited the accuracy to 10%, but this is not a fundamental limitation in the method. Values of the thermal emissivity are also obtained from the rate of change of sample temperature.