Paired emitter-detector diodes (PEDDs) coupled with potentiometric pH-meter or multimeter have been characterized as complete instruments for common photometric measurements as well as optical flow-through detectors. The analytical characteristics of investigated devices have been illustrated by the use of two paired red LEDs for the determination of bromothymol blue (BTB) as a model analyte. The developed devices allow rapid and sensitive BTB detection in the ppm range of concentration. Measurements with pH-meter resulted in a wide linearity of PEDD response, whereas PEDDs coupled with ordinary voltmeter offer a significant enhancement of sensitivity. Some practical applications of developed detector for the analytical and bioanalytical measurements have been indicated.