This paper discusses the influence of electron scavengers (such as oxygen) on the emission properties of colloidal suspensions of nanocrystalline ZnO particles. Emission measurements are performed on deaerated suspensions of ZnO particles in alcohols (2-propanol and ethanol) and other organic solvents (DMF and propylene carbonate). Upon UV irradiation the visible emission quenches while the intensity of the exciton emission increases. Admission of oxygen into the colloidal suspension restores the initial emission properties. The role of oxygen in the quenching of the visible emission is accounted for by assuming that oxygen scavenges photogenerated electrons. In the absence of oxygen, excess photogenerated electrons are present resulting in a passivation of V-O(.)/V-O(..) centers, which are involved in the visible emission process in ZnO.