AbstractTransparent nanocrystalline ZnO films were prepared from colloidal ZnO solutions containing particles of different sizes. The colloidal solution as well as the films exhibit size quantization effects. The absorption and stationary and time resolved fluorescence of the films as well as the photocurrent strongly depend on the applied potential. The electrochemical and spectroscopic investigations allow direct measurement of the quantum mechanical confinement energy for electrons in the nanocrystalline particles of the films. The optical properties and the charge carrier transport through the films are explained by a model involving electronic levels of the individual particles.