We present the derivation of the Midcourse Space Experiment (MSX) SPatial InfRared Imaging Telescope (SPIRIT III) ‘‘truth’’ inband irradiances and associated uncertainties for the six primary calibration stars in the six IR radiometer bands. The inband irradiances are calculated from the approved stellar spectral irradiances S(?) from the literature and the measured end-to-end relative spectral response R(?) curves for the bands. The spectral uncertainties associated with the stellar spectra and the instrument response are propagated to provide total uncertainties for the inband irradiance values.