The responsivities of the Landsat-5 Thematic Mapper (TM) reflective bands are characterized over the lifetime of the instrument using its internal calibration system. This system illuminates only the focal planes and aft optics of the TM so that it does not capture changes in the telescope. The observed changes are quantified and categorized as to whether they are likely to be true instrument responsivity changes or changes in the internal calibrator system itself. Changes observed that are likely to be true instrument changes are: (1) 7 percent, 5 percent, 8 percent and 7 percent exponential-like decreases in responsivity with decay half lives of 250, 180, 60 and 110 days in bands 1 to 4, respectively, during the initial on-orbit period and (2) an oscillation in response of about 5 percent peak-to-peak in bands 5 and 7. The first effect is believed to originate in the TM spectral bandpass filters and the second effect is believed to be due to an icing build up in the cold focal plane window. Two rapid apparent responsivity changes, one a decrease and one an increase, which are peculiar to particular internal calibrator lamps are believed to be due to changes in the lamp assemblies themselves as is a gradual increase in all detector's responsivities with time. An annual oscillation of up to 2 percent peak-to-peak in all bands is likely the product of both a temperature sensitivity of the IC and the TM primary focal plane.