A general method of accounting for emissivity in making temperature determinations of greybody surfaces from radiometric data is presented. The method differs from previous treatments in that a simple blackbody calibration and graphical approach is used rather than numerical integrations which require detailed knowledge of an instrument's spectral characteristics. Also, errors caused by approximating instrumental response with the Stephan-Boltzman law rather than with an appropriately weighted Planck integral are examined. In the 8–14 μm wavelength interval, it is shown that errors are at most on the order of 3°C for the extremes of the earth's temperature and emissivity. For more practical limits, however, errors are less than 0.5°C.