Radiative transfer within an interferometric spectrometer is discussed in relation to measurements of infrared transmittance spectra of gases. It is shown that for certain types of interferometric spectrometers, radiation from sources other than the spectrometer source must be considered, including the gas sample itself, if significant errors in measurement of line strengths and line widths are to be avoided. The validity of the radiative transfer theory is demonstrated by applying it to spectra of an evacuated gas cell, recorded over a range of temperatures, and thereby deriving a temperature independent emissivity for the cell windows. The error introduced into measurements of spectral line strengths and line widths is discussed with reference to experimental values of these parameters obtained for lines in the v2 band of H2O using a Bomem DA 3.002 spectrometer. Ways in which the effects of radiation from the gas cell and its contents can be minimised or eliminated are discussed, including an experimental procedure involving measurements with two different temperatures of the spectrometer source.