Quenching of the photoluminescence of colloidal zinc oxide nanocrystals by a series of stable nitronyl nitroxide radicals was studied by means of stationary and time-resolved luminescence spectroscopy. Among the studied radicals the most efficient quenchers of the ZnO luminescence are the carboxylsubstituted species. The meta-substituted radical was found to be a more active quencher, than parasubstituted one due to a closer proximity of the radical center to the nanocrystals surface. The EL quenching has a complex dynamic/static character. The dynamic quenching arises from photocatalytic radical reduction by ZnO conduction band electrons, while the static quenching is caused by adsorption of the photoreduction products on the nanocrystal surface. The non-substituted and OH-substituted radicals are inferior to the products of their photoreduction in capability of adsorption of the ZnO surface, and the quenching is dominated by interactions between the nanocrystals and photoreduced hydroxylamines. In case of COOH-substituted radicals, however, the radicals compete with the photoreduction products for the surface sites of ZnO nanocrystals resulting in a dynamic character of photoluminescence quenching.