The continuous growth in the demand of good quality water for different purposes makes the control of environmental pollution a matter for public and official concern. The monitoring of particles for quality control of water or wastewater treatment is becoming important and the need for the development of control devices is increasing. The purpose of this work was to develop a photosedimentation method for floc characterization combining gravitational settling with photoelectric measurements. Settling particles cross a light beam that illuminates a detection system. The attenuation of the beam is related to the projected area of the particles and so the particle size distribution can be determined. When a floc crosses the beam, the amount of light reaching the detector is reduced. Voltage/time signals are analyzed in order to produce the physical characteristics of flocs such as size and density. Different light sources and detection systems were tested in calibration tests and afterwards flocculated suspensions were used for experimental analysis. Floc characteristics could then be related to the experimental conditions of floc formation (mixing conditions and polymer concentration). With the experience acquired, a prototype was designed and implemented, being capable of operating in laboratory or field conditions.