The automated techniques of measurement of water parameters are developed based on flow analysis (flow injection analysis, sequential injection analysis). PEDD detectors (paired emitter–detector diode), built with the use of two paired light-emitting diodes (LEDs) – emission and detection, can serve that purpose. The aim of the studies was to optimize the system for determination of the dissolved organic substances in coloured water with the use of PEDDs in the flowing-stream measurement system. The investigations were conducted for a number of concentrations of the humic, fulvic and tannic acids, which were measured as the dissolved organic carbon (DOC). Three arrangements of diodes (PEDD) with a selected emission spectrum and absorption characteristics were used. A two-beam flow cell was designed for the measurements. The signal from the detection diode was recorded and converted into absorbance through software written for that purpose. The results obtained showed a high sensitivity in relation to the concentration as well as the selectivity in the type of substances studied. A miniature and cheap photometric flow analysis set was proposed, adapted to work with one or several light wavelengths and steered by a microprocessor unit.