We combine optical absorption spectroscopy, ex situ transmission electron microscopy (TEM) imaging, and variable-temperature measurements to study the effect of ultraviolet (UV) light and heat treatments on ethanedithiol-treated PbS quantum dot (QD) films as a function of ambient atmosphere, temperature, and QD size. Film aging occurs mainly by oxidation or ripening and sintering depending on QD size and the presence of oxygen. We can stop QD oxidation and greatly suppress ripening by infilling the films with amorphous Al(2)O(3) using room-temperature atomic layer deposition (ALD).