Gradual deterioration of calibration of the Advanced Very High Resolution Radiometer (AVHRR) can be explained by the erosion of the scanning mirror as a result of interaction with interplanetary dust. Other elements of the instrument (optical filters, detector, electronics) were also examined for their behavior in space conditions and were found most likely to be stable and not responsible for the decrease in sensitivity. Calculations show that the erosion of the scanning mirror is of the order of 4.5% per year, although this erosion is very variable because of the variability of the density of interplanetary dust. This erosion can easily explain the 3-5% per year deterioration of theAVHRR sensors on NOAA-7 and NOAA-9.