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 used 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. In this report, we study the feasibility of using a LED as a luminescent detector and realising a sensing instrument whose small size possibles to embed it into a portable measurement system. A complete study of instrument was carried out in order to optimise the specifications of the portable instrument such as: range, sensitivity, short term and long term stability, dynamical behaviour, temperature influence, humidity influence and temporal drift. Keywords: Oxygen sensor, Gas sensor, Optical sensor, Paired emitter detector-diode sensor; Portable instrumentation.