The changes in the free-carrier concentration in polycrystalline ZnO films during exposure to H2 and O2 plasmas were studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. The carrier concentration and mobility were extracted from the free-carrier absorption in the infrared using a model for the dielectric function. The electron density in polycrystalline zinc oxide films may be significantly increased by >1019 cm−3 by brief exposures to hydrogen plasma at room temperature and decreased by exposure to O2 plasmas. Room-temperature oxygen plasma removes a fraction of the H at donor sites but both elevated temperatures ( ∼ 225 °C) and O2 plasma were required to remove the rest. We demonstrate that combinations of O2 and H2 plasma treatments can be used to manipulate the carrier density in ZnO films. However, we also show the existence of significant drifts ( ∼ 15%) in the carrier concentrations over very long time scales (hours). Possible sites for H incorporation in polycrystalline films and reasons for the observed carrier-concentration changes are proposed.