Method for determining the UV transmittance of water in a UV disinfection plant, through which water flows, wherein the UV disinfection plant has a plurality of radiator arrangements, each with a UV radiation source, a sleeve tube which surrounds the UV radiation source and which has an end face at an open end, and with a UV-C sensor which detects the UV radiation emerging from the sleeve tube without the influence of the water, and with at least one further UV sensor which is arranged at a distance from the sleeve tubes of the radiator arrangements, wherein the method includes the following steps: measuring the UV radiant power emerging from the sleeve tube; measuring an amount of the transmitted radiant power by the further UV sensor; and determining the transmittance of the water by an amount of the emerged radiant power and of the transmitted radiant power.