We have quantitatively investigated the charging of colloidal ZnO films by cyclic voltammetry and potential step measurements. Up to six electrons can be stored on each colloidal particle of the film (mean particle diameter 5.9 nm) before zinc reduction occurs. Fluorescence and absorbance of the films are already strongly influenced in potential regions where less than one electron is stored on each particle. Potentiostatically controlled and time-resolved photocurrent measurements show the influence of trap filling as well as the role of the electrolyte on the charge transport through the films.