The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of five scientific instruments onboard the NASA EOS Terra spacecraft, launched December 1999. Early on-orbit calibration and characterization of Terra MODIS showed noticeable mirror side dependent degradation, particularly in the visible spectral region through the use of its on-board calibrators (OBC). These calibrators, including a solar diffuser (SD) and solar diffuser stability monitor (SDSM) system and a spectro-radiometric calibration assembly (SRCA), are operated at either weekly or monthly frequency, tracking the sensor"s calibration stability and updating calibration coefficients. This paper describes an approach developed shortly after Terra"s launch which studies MODIS reflective solar bands (RSB) mirror side dependent degradation and presents its on-orbit time series trending results. The approach uses simultaneous Earth scene observations from MODIS and the Multi-angle Imaging Spectroradiometer (MISR) on the Terra spacecraft. To reduce the effects due to Earth scene variations, only MODIS/MISR radiance or response ratios averaged over many spatially and spectrally matched pixels are used in the analysis and support for the calibration. Our results show that over a five-year period MODIS detector response ratios of mirror side 2 relative to mirror side 1 degraded 7.0%, 5.0% and 2.5% in bands 8 (0.41μm), 9 (0.44μm) and 3 (0.47μm) at the nadir view. The change in the mirror side ratio is much smaller for bands at longer wavelengths. The results also show that the change in the mirror side ratio is strongly dependent on the viewing angle. The impact of the degradation and response versus scan angle corrections to the RSB calibration is discussed.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.