The spatial infrared imaging telescope (SPIRIT III) radiometer is the primary instrument aboard the Midcourse Space Experiment (MSX), which was launched on April 24, 1996. The SPIRIT III radiometer observed various orbital and suborbital targets, celestial sources, zodiacal emissions, the earth’s airglow, the aurora, and other upper atmospheric phenomena until its cryogen was depleted on February 26, 1997. During its on-orbit mission, the temperature of the radiometer focal planes rose as the cryogen was depleted, which resulted in expected changes in the radiometer’s responsivity. The responsivity’s dependency on temperature was quantified from ground calibration experiments to determine correction functions. These correction functions were verified by applying them to on-orbit observations of internal stimulators. The characterization of the dependence of responsivity on focal plane temperature is presented, the temperature effects as a function of integration mode and response level are described, and the results of applying these corrections to on-orbit internal stimulator data are given.