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 including 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 luminescent detector is studied. Its small size allows the integration of the instrument into a portable measurement system. A systematic study of the system capabilities as a portable instrument, was carried out in order to optimize: range, sensitivity, short term and long term stability, dynamical behavior, temperature influence, humidity influence and temporal drift.