A portable instrument for oxygen determination, based on the quenching of phosphorescent octaethylporphyrin by gaseous O2, has been developed using the fluorimetric paired emitter–detector diode technique (FPEDD). The instrument configuration consists of two light-emitting diodes (LEDs) facing each other, with an interchangeable support containing a phosphorescent membrane in between, in which one of the LEDs is used as the light source (emitter LED) and the other, working in reverse bias mode, as the light detector. The feasibility of using a LED as a luminescence detector is studied. Its small size enables integration of the instrument into a portable measurement system. A systematic study of the system capabilities as a portable instrument was performed to optimize range, sensitivity, short term and long term stability, dynamic behaviour, effect of temperature and humidity, and temporal drift.