There is described a method of determining the UV fluence received by a fluid. The method comprises the steps of: (a) irradiating the fluid at an unknown UV fluence; (b) measuring the fluorescence of a test sample of the fluid after irradiation in Step (a) to produce a test signal proportional to the concentration of a prescribed fluorescent composition of matter comprised in the test sample; and (c) determining the value of the unknown UV fluence by comparing the test signal to a calibration curve of a control signal proportional to concentration of the prescribed fluorescent composition of matter in the fluid as a function of applied UV fluence. There is also described a system for determining the UV fluence received by a fluid being treated in UV fluid treatment system comprising at least one UV source. The system comprises: (a) a radiation-transparent vessel for receiving a test sample of the fluid after irradiation of the fluid at an unknown UV fluence; (b) a fluorometer for measuring the fluorescence of the test sample received in the radiation-transparent vessel to produce a test signal proportional to the concentration of a prescribed fluorescent composition of matter comprised in the test sample; and (c) a controller configured to determine the value of the unknown UV fluence by comparing the test signal to a calibration curve of a control signal proportional to concentration of the prescribed fluorescent composition of matter in the fluid as a function of applied UV fluence.