A device for determining the composition of a mixture of fluids that flow along a pipe includes: a radiation source for illuminating the mixture with radiation; a detector for detecting radiation that has been attenuated by the mixture; and a device for monitoring the flow rate of fluid along the pipe and outputting a signal indicative of the flow rate. The device for determining further includes a device for adjusting the intensity of radiation emitting by the radiation source in response to the signal indicative of the flow rate so that the intensity of the radiation source is reduced if the flow rate reduces.