The development of an online sensor to determine the fouling properties of aqueous process fluids is described. A plastic optical fiber with its cladding removed over a sensitized length measures the growth of biofilms in a closed loop water process system by evanescent field attenuation and intensity modulation. The sensor detects material build-up at the core-cladding interface by means of refractive index modulation. A theoretical model is developed showing that an increase in cladding refractive index reduces the intensity of light propagating in the fiber and attenuates the high order modes. The modulation mechanism of the sensor is demonstrated using a CCD camera and frame grabber to record the far field modal distribution of the fiber, when the outer modes are excited. The intensity distribution changes spatially in response to the biofilm deposit on the sensor, indicating evanescent field attenuation.