This paper discusses the operational in-orbit GOES-8 and GOES-10 imager scan-mirror emissivity trends, as well as their diurnal cycles. The imagers (and sounders) aboard both GOES-8 and GOES-10 experience a variation in scan-mirror emissivity along the east-west scan direction. The most obvious manifestation of this phenomenon is a difference in output between the east and west sides when the insthiments view space, but it is also present in observations of the Earth. The phenomenon is accounted for in the calibration process with an algorithm that makes use ofcoefficients incorporating the variation ofthe scanmirror emissivity with east-west scan angle. The coefficients are derived from measurements of space above the north pole and below the south pole made during GOES station-keeping maneuvers, which are performed a few times a year. Over time, these measurements allow us to compile a trend ofthe east-westemissivityvariation. Operational full-disk images are used to diagnose the diurnal behavior of the residual (after correction) east-west output differences. A comparison between the scan-mirror emissivity of GOES-8 and that of GOES-10 is made to search for patterns related to specific satellites. This paper also reviews how the east-west scan-mirror emissivity coefficients are derived and evaluates the effect ofuncertainties in the band-averaged emissivity measurements on the GOES calibration. An effective scan-mirror temperature is proposed to minimize the residual east-west output differences.