This short note describes a newly developed device based on the use of an endoscope coupled to a CCD camera to measure in situ particle sizes. It was initially designed for river studies with lower water depths and turbidities, and further adapted for estuarine studies to participate in the intercalibration programme organized in the Elbe estuary. Major results reported here confirm that most suspended matter has diameters in the range of several hundred micrometres. The upper size limit of our device (400 µm) was frequently exceeded for individual particles. A less magnifying lens could be used to overcome this problem, but then smaller sizes would not be detected.