The emissivity errors of total, partial and monochromatic radiation pyrometers for use in the temperature range 500-800° K are discussed theoretically. The pyrometer output is expressed as a power law of absolute temperature with an exponent n which varies with temperature and the spectral response of the pyrometer. The emissivity error decreases as n increases. It is shown that n increases as the spectral response of the pyrometer is restricted to shorter wavelengths in the near infra-red region.