We examine in detail the calibration procedures for the advanced very high resolution radiometer (AVHRR) infrared channels. The AVHRR literature states that the linear mapping used to convert radiometric count to scene radiance is satisfactory for channel 3, but is only approximately correct for channels 4 and 5 since these channels have a slight curvature in their respective radiance-versus-count response functions, so a nonlinearity correction (NLC) is applied. Traditionally, these adjustments have been tabulated as brightness temperature corrections; we find that when expressed as radiance corrections, the NLCs can be written in a particularly simple form which depends only on the temperature of the internal calibration target. This new formulation eliminates the need to interpolate within the tables of temperature NLCs and permits the correction to be applied prior to conversion to equivalent brightness temperature, rather than after. Examination of the calibration records for 48 NOAA 11 passes sampled over a full year revealed several significant features: (1) For low Sun elevation angles relative to the satellite, sunlight can apparently leak into the optical path of the AVHRR, distorting the in-flight calibration data for channel 3, producing errors in apparent scene temperature as large as 2¿C. (2) There is a clear demarcation between the calibration slopes for day and night operation of the satellite. (3) The thermal step that occurs when the spacecraft crosses the night-day boundary into full sunlight leads to transient calibration errors which decay to zero with a time constant of about 40 s. At the peak of the transient, the apparent scene temperature can be depressed by over 1¿C in channel 3, and by up to 0.4¿C in channels 4 and 5. Quality control measures and techniques for compensating for calibration problems are discussed. ¿ American Geophysical Union 1992