We have identified a first-wafer effect for photoresist ashing and silicon nitride-polysilicon stripping in remote plasma reactors. The first-wafer effect consists of the first wafer etching differently from the subsequent wafers in a lot. For photoresist ashing, the first wafer ashes faster than subsequent wafers. For silicon nitride and polysilicon stripping, first wafers show higher etch rates of silicon nitride and polysilicon, while silicon dioxide first wafers etch faster for the polysilicon strip process, and slower for the silicon nitride strip process. We have modeled the first-wafer effect for photoresist ashing. We found an inverse relationship between the percentage change in the time to clear the photoresist from the wafer and the time delay between processing sequential wafers. We have included this first-wafer effect in the on-line statistical process control strategy for the photoresist asher in our laboratory. Examination of this first-wafer effect suggests that it may be caused by the generation of species in the discharge in the first few seconds of operation that alter the reactivity of the chamber walls. While these species are quick to adsorb on the walls, they only desorb slowly. Pumping on the chamber in the absence of a microwave discharge returns the chamber to its original state.