We report the effects of N2 and O2 on the electrical properties of PbSe quantum-dot (QD) films treated with 1,2-ethanedithiol (EDT) by measuring the changes in the current-voltage characteristics of QD field-effect transistors (FETs). EDT-treated PbSe QD films at a base pressure of ∼10-5 Torr exhibit ambipolar transport. Exposing these films to N2 shifts the transfer characteristics toward negative gate-voltage values and increases the electron mobility. These changes could be reversed entirely by removing the N2 gas over the FET and returning to base pressure. Oxygen exposure shifts the transfer characteristics in the opposite direction toward positive gate-voltage values. Moreover, oxygen exposure reduces charge mobility but increases film conductivity. For exposures up to ∼108 langmuir, these O2-induced changes could be reversed completely by removing the O2 gas over the sample and returning to base pressure. However, after ∼1010 langmuir of O2 exposure, the changes are irreversible. The QD films then permanently become p-type and the decrease in charge mobility remains even after returning to base pressure.